[samr21] Add possibility to define board with command line (#3491)

By default Makefile-samr21 builds for SAM R21 Xplained Pro board. Another
board can be specified with BOARD variable through command line:

    make -f examples/Makefile-samr21 BOARD=<TARGET_BOARD>

Currently supported following boards: SAMR21_XPLAINED_PRO,
SAMR21G18_MODULE, SAMR21B18_MODULE.

Signed-off-by: Oleksandr Grytsov <[email protected]>

examples/Makefile-samr21

@@ -41,6 +41,21 @@ OBJCOPY                         = arm-none-eabi-objcopy
 
 BuildJobs                      ?= 10
 
+
+# Board setup
+
+BOARD ?= SAMR21_XPLAINED_PRO
+
+ifeq ($(BOARD), SAMR21_XPLAINED_PRO)
+    CPU = __SAMR21G18A__
+else ifeq ($(BOARD), SAMR21G18_MODULE)
+    CPU = __SAMR21G18A__
+else ifeq ($(BOARD), SAMR21B18_MODULE)
+    CPU=__SAMR21G18A__
+else
+    $(error Invalid BOARD=$(BOARD) (currently supported SAMR21_XPLAINED_PRO, SAMR21G18_MODULE, SAMR21B18_MODULE))
+endif
+
 configure_OPTIONS               = \
     --enable-cli                  \
     --enable-diag                 \
@@ -60,6 +75,8 @@ CONFIG_FILE      = OPENTHREAD_PROJECT_CORE_CONFIG_FILE='\"openthread-core-samr21
 CONFIG_FILE_PATH = $(AbsTopSourceDir)/examples/platforms/samr21/
 
 COMMONCFLAGS                   := \
+    -DBOARD=$(BOARD)              \
+    -D$(CPU)                      \
     -fdata-sections               \
     -ffunction-sections           \
     -Os                           \

examples/platforms/samr21/Makefile.am

@@ -33,16 +33,8 @@ lib_LIBRARIES = libopenthread-samr21.a
 override CFLAGS                              := $(filter-out -pedantic-errors,$(CFLAGS))
 override CXXFLAGS                            := $(filter-out -pedantic-errors,$(CXXFLAGS))
 
-#SAMR21_BOARD=USER_BOARD
-#SAMR21_CPU=__SAMR21E18A__
-
-SAMR21_BOARD=SAMR21_XPLAINED_PRO
-SAMR21_CPU=__SAMR21G18A__
-
 libopenthread_samr21_a_CPPFLAGS                                                                              = \
     -D ARM_MATH_CM0PLUS=true                                                                                   \
-    -D BOARD=$(SAMR21_BOARD)                                                                                   \
-    -D $(SAMR21_CPU)                                                                                           \
     -D PHY_AT86RF233                                                                                           \
     -D SAL_TYPE=AT86RF2xx                                                                                      \
     -D USART_CALLBACK_MODE=true                                                                                \