Makefile.include: introduce 'BOARDSDIR' for boards directory

Introduce a new variable 'BOARDSDIR' to use when referencing the base
boards directory.

This is a transition to allow defining external boards while still using
the `RIOT/boards` directory for like `boards/common` for example through
'RIOTBOARD'.

Makefile.include

@@ -25,6 +25,7 @@ include $(RIOT_MAKEFILES_GLOBAL_PRE)
 RIOTBASE       ?= $(_riotbase)
 RIOTCPU        ?= $(RIOTBASE)/cpu
 RIOTBOARD      ?= $(RIOTBASE)/boards
+BOARDSDIR      ?= $(RIOTBOARD)
 RIOTMAKE       ?= $(RIOTBASE)/makefiles
 RIOTPKG        ?= $(RIOTBASE)/pkg
 RIOTTOOLS      ?= $(RIOTBASE)/dist/tools
@@ -81,6 +82,13 @@ override BINDIR         := $(abspath $(BINDIR))
 override PKGDIRBASE     := $(abspath $(PKGDIRBASE))
 override DLCACHE_DIR    := $(abspath $(DLCACHE_DIR))
 
+# Keep standard make behavior for new variables
+__DIRECTORY_VARIABLES += \
+  BOARDSDIR \
+  #
+
+BOARDSDIR               := $(abspath $(BOARDSDIR))
+
 # Ensure that all directories are set and don't contain spaces.
 ifneq (, $(filter-out 1, $(foreach v,$(__DIRECTORY_VARIABLES),$(words $($(v))))))
   $(info Aborting compilation for your safety.)

makefiles/vars.inc.mk

@@ -26,6 +26,7 @@ export APPDEPS               # Files / Makefile targets that need to be created
 export RIOTBASE              # The root folder of RIOT. The folder where this very file lives in.
 export RIOTCPU               # For third party CPUs this folder is the base of the CPUs.
 export RIOTBOARD             # For third party BOARDs this folder is the base of the BOARDs.
+export BOARDSDIR             # For third party BOARDs this folder is the base of the BOARDs. (Migration in progress)
 export RIOTPKG               # For overriding RIOT's pkg directory
 export RIOTTOOLS             # Location of host machine tools
 export RIOTPROJECT           # Top level git root of the project being built, or PWD if not a git repository