diff --git a/boards/arduino-leonardo/Makefile.include b/boards/arduino-leonardo/Makefile.include
index 96cfb4faaa..886a694c23 100644
--- a/boards/arduino-leonardo/Makefile.include
+++ b/boards/arduino-leonardo/Makefile.include
@@ -1,7 +1,6 @@
 # define the cpu used by the arduino uno board
 export CPU = atmega32u4
 
-# export needed for flash rule
 PORT_LINUX      ?= /dev/ttyUSB0
 AVRDUDE_PORT    ?= /dev/ttyACM0
 PORT_DARWIN     ?= $(firstword $(sort $(wildcard /dev/tty.usbmodem*)))
diff --git a/boards/common/arduino-mkr/Makefile.include b/boards/common/arduino-mkr/Makefile.include
index bb0417cd0b..d0b98e8ec8 100644
--- a/boards/common/arduino-mkr/Makefile.include
+++ b/boards/common/arduino-mkr/Makefile.include
@@ -2,9 +2,8 @@
 export CPU = samd21
 export CPU_MODEL = samd21g18a
 
-#export needed for flash rule
-export PORT_LINUX ?= /dev/ttyACM0
-export PORT_DARWIN ?= $(firstword $(sort $(wildcard /dev/tty.usbmodem*)))
+PORT_LINUX ?= /dev/ttyACM0
+PORT_DARWIN ?= $(firstword $(sort $(wildcard /dev/tty.usbmodem*)))
 
 # setup serial terminal
 include $(RIOTMAKE)/tools/serial.inc.mk
diff --git a/boards/feather-m0/Makefile.include b/boards/feather-m0/Makefile.include
index 4243a2e659..1379586a70 100644
--- a/boards/feather-m0/Makefile.include
+++ b/boards/feather-m0/Makefile.include
@@ -2,9 +2,8 @@
 export CPU = samd21
 export CPU_MODEL = samd21g18a
 
-#export needed for flash rule
-export PORT_LINUX ?= /dev/ttyACM0
-export PORT_DARWIN ?= $(firstword $(sort $(wildcard /dev/tty.usbmodem*)))
+PORT_LINUX ?= /dev/ttyACM0
+PORT_DARWIN ?= $(firstword $(sort $(wildcard /dev/tty.usbmodem*)))
 
 # setup serial terminal
 include $(RIOTMAKE)/tools/serial.inc.mk
diff --git a/boards/sensebox_samd21/Makefile.include b/boards/sensebox_samd21/Makefile.include
index 8d1d48ee6f..57521f1f57 100644
--- a/boards/sensebox_samd21/Makefile.include
+++ b/boards/sensebox_samd21/Makefile.include
@@ -2,9 +2,8 @@
 export CPU = samd21
 export CPU_MODEL = samd21g18a
 
-#export needed for flash rule
-export PORT_LINUX ?= /dev/ttyACM0
-export PORT_DARWIN ?= $(firstword $(sort $(wildcard /dev/tty.usbmodem*)))
+PORT_LINUX ?= /dev/ttyACM0
+PORT_DARWIN ?= $(firstword $(sort $(wildcard /dev/tty.usbmodem*)))
 
 # setup serial terminal
 include $(RIOTMAKE)/tools/serial.inc.mk
diff --git a/boards/sodaq-autonomo/Makefile.include b/boards/sodaq-autonomo/Makefile.include
index e8ecddab61..a29d9ee745 100644
--- a/boards/sodaq-autonomo/Makefile.include
+++ b/boards/sodaq-autonomo/Makefile.include
@@ -2,9 +2,8 @@
 export CPU = samd21
 export CPU_MODEL = samd21j18a
 
-#export needed for flash rule
-export PORT_LINUX ?= /dev/ttyACM0
-export PORT_DARWIN ?= $(firstword $(sort $(wildcard /dev/tty.usbmodem*)))
+PORT_LINUX ?= /dev/ttyACM0
+PORT_DARWIN ?= $(firstword $(sort $(wildcard /dev/tty.usbmodem*)))
 
 # setup serial terminal
 include $(RIOTMAKE)/tools/serial.inc.mk
diff --git a/boards/sodaq-explorer/Makefile.include b/boards/sodaq-explorer/Makefile.include
index 9dc1d6fc17..db42ef2027 100644
--- a/boards/sodaq-explorer/Makefile.include
+++ b/boards/sodaq-explorer/Makefile.include
@@ -2,9 +2,8 @@
 export CPU = samd21
 export CPU_MODEL = samd21j18a
 
-#export needed for flash rule
-export PORT_LINUX ?= /dev/ttyACM0
-export PORT_DARWIN ?= $(firstword $(sort $(wildcard /dev/tty.usbmodem*)))
+PORT_LINUX ?= /dev/ttyACM0
+PORT_DARWIN ?= $(firstword $(sort $(wildcard /dev/tty.usbmodem*)))
 
 # setup serial terminal
 include $(RIOTMAKE)/tools/serial.inc.mk
diff --git a/boards/sodaq-one/Makefile.include b/boards/sodaq-one/Makefile.include
index 95366d62a2..4b87c5c8cb 100644
--- a/boards/sodaq-one/Makefile.include
+++ b/boards/sodaq-one/Makefile.include
@@ -2,9 +2,8 @@
 export CPU = samd21
 export CPU_MODEL = samd21g18a
 
-#export needed for flash rule
-export PORT_LINUX ?= /dev/ttyACM0
-export PORT_DARWIN ?= $(firstword $(sort $(wildcard /dev/tty.usbmodem*)))
+PORT_LINUX ?= /dev/ttyACM0
+PORT_DARWIN ?= $(firstword $(sort $(wildcard /dev/tty.usbmodem*)))
 
 # setup serial terminal
 include $(RIOTMAKE)/tools/serial.inc.mk
diff --git a/boards/sodaq-sara-aff/Makefile.include b/boards/sodaq-sara-aff/Makefile.include
index ed528f1bea..70077c179a 100644
--- a/boards/sodaq-sara-aff/Makefile.include
+++ b/boards/sodaq-sara-aff/Makefile.include
@@ -2,9 +2,8 @@
 export CPU = samd21
 export CPU_MODEL = samd21j18a
 
-#export needed for flash rule
-export PORT_LINUX ?= /dev/ttyACM0
-export PORT_DARWIN ?= $(firstword $(sort $(wildcard /dev/tty.usbmodem*)))
+PORT_LINUX ?= /dev/ttyACM0
+PORT_DARWIN ?= $(firstword $(sort $(wildcard /dev/tty.usbmodem*)))
 
 # setup serial terminal
 include $(RIOTMAKE)/tools/serial.inc.mk
diff --git a/dist/tools/buildsystem_sanity_check/check.sh b/dist/tools/buildsystem_sanity_check/check.sh
index f108fe3214..9d0a7bbaa8 100755
--- a/dist/tools/buildsystem_sanity_check/check.sh
+++ b/dist/tools/buildsystem_sanity_check/check.sh
@@ -58,6 +58,7 @@ UNEXPORTED_VARIABLES+=('PREFLASHER' 'PREFFLAGS' 'FLASHDEPS')
 UNEXPORTED_VARIABLES+=('DEBUG_ADAPTER' 'DEBUG_ADAPTER_ID')
 UNEXPORTED_VARIABLES+=('PROGRAMMER_SERIAL')
 UNEXPORTED_VARIABLES+=('STLINK_VERSION')
+UNEXPORTED_VARIABLES+=('PORT_LINUX' 'PORT_DARWIN')
 
 EXPORTED_VARIABLES_ONLY_IN_VARS=()
 check_not_exporting_variables() {