cpu/Kconfig: Declare common CPU symbols

The symbols used to define a CPU are:
- CPU
- CPU_MODEL
- CPU_FAMILY
- CPU_ARCH

Kconfig

@@ -14,6 +14,7 @@ orsource "$(RIOTBOARD)/$(BOARD)/Kconfig"
 orsource "$(RIOTCPU)/$(CPU)/Kconfig"
 
 rsource "$(RIOTBOARD)/Kconfig"
+rsource "$(RIOTCPU)/Kconfig"
 
 # The application may declare new symbols as well
 osource "$(APPDIR)/Kconfig"

cpu/Kconfig

@@ -0,0 +1,48 @@
+# Copyright (c) 2020 HAW Hamburg
+#
+# This file is subject to the terms and conditions of the GNU Lesser
+# General Public License v2.1. See the file LICENSE in the top level
+# directory for more details.
+#
+#   +-----------+
+#   | CPU_MODEL |
+#   +-----------+
+#        || selects
+#        \/
+#  +------------+
+#  | CPU_SERIES |
+#  +------------+
+#        || selects
+#        \/
+#  +------------+
+#  | CPU_FAMILY |
+#  +------------+
+#        || selects
+#        \/
+#   +----------+
+#   | CPU_ARCH |
+#   +----------+
+config CPU
+    string
+    help
+        Name of the currently selected CPU.
+
+config CPU_MODEL
+    string
+    help
+        Model of the currently selected CPU.
+
+config CPU_SERIES
+    string
+    help
+        Series of the currently selected CPU.
+
+config CPU_FAMILY
+    string
+    help
+        Family of the currently selected CPU.
+
+config CPU_ARCH
+    string
+    help
+        Architecture of the currently selected CPU.