nrf/Makefile: Allow external board definitions.

Trying to use an external board definition according to
https://github.com/micropython/micropython-example-boards on the nrf port
failed with "Invalid BOARD specified".  Replacing all ocurrences of
"boards/$(BOARD)" with "$(BOARD_DIR)" following the example of
stm32/Makefile fixes that.

Signed-off-by: Christian Walther <cwalther@gmx.ch>

ports/nrf/Makefile

@@ -9,8 +9,8 @@ BOARD ?= PCA10040
 BOARD_DIR ?= boards/$(BOARD)
 endif
 
-ifeq ($(wildcard boards/$(BOARD)/.),)
-$(error Invalid BOARD specified)
+ifeq ($(wildcard $(BOARD_DIR)/.),)
+$(error Invalid BOARD specified: $(BOARD_DIR))
 endif
 
 # If SoftDevice is selected, try to use that one.
@@ -19,7 +19,7 @@ SD_LOWER = $(shell echo $(SD) | tr '[:upper:]' '[:lower:]')
 
 # TODO: Verify that it is a valid target.
 
-include boards/$(BOARD)/mpconfigboard.mk
+include $(BOARD_DIR)/mpconfigboard.mk
 
 ifeq ($(SD), )
 	# If the build directory is not given, make it reflect the board name.
@@ -48,7 +48,7 @@ ifneq ($(LD_FILE),)
 	LD_FILES = $(LD_FILE)
 endif
 
--include boards/$(BOARD)/modules/boardmodules.mk
+-include $(BOARD_DIR)/modules/boardmodules.mk
 
 # qstr definitions (must come before including py.mk)
 QSTR_DEFS = qstrdefsport.h
@@ -113,7 +113,7 @@ NRF_DEFINES += -D$(MCU_SUB_VARIANT_UPPER)
 NRF_DEFINES += -DCONFIG_GPIO_AS_PINRESET
 
 MAKE_PINS = boards/make-pins.py
-BOARD_PINS = boards/$(BOARD)/pins.csv
+BOARD_PINS = $(BOARD_DIR)/pins.csv
 AF_FILE = $(MCU_VARIANT)_af.csv
 PREFIX_FILE = boards/$(MCU_VARIANT)_prefix.c
 GEN_PINS_SRC = $(BUILD)/pins_gen.c
@@ -139,7 +139,7 @@ endif
 CFLAGS += $(CFLAGS_MCU_$(MCU_SERIES))
 CFLAGS += $(INC) -Wall -Werror -ansi -std=c11 -nostdlib $(COPT) $(NRF_DEFINES) $(CFLAGS_EXTRA)
 CFLAGS += -fno-strict-aliasing
-CFLAGS += -Iboards/$(BOARD)
+CFLAGS += -I$(BOARD_DIR)
 CFLAGS += -DNRF5_HAL_H='<$(MCU_VARIANT)_hal.h>'
 
 LDFLAGS += $(CFLAGS)
@@ -481,7 +481,7 @@ $(OBJ): | $(HEADER_BUILD)/pins.h
 
 # Use a pattern rule here so that make will only call make-pins.py once to make
 # both pins_gen.c and pins.h
-$(BUILD)/%_gen.c $(HEADER_BUILD)/%.h $(HEADER_BUILD)/%_af_const.h: boards/$(BOARD)/%.csv $(MAKE_PINS) $(AF_FILE) $(PREFIX_FILE) | $(HEADER_BUILD)
+$(BUILD)/%_gen.c $(HEADER_BUILD)/%.h $(HEADER_BUILD)/%_af_const.h: $(BOARD_DIR)/%.csv $(MAKE_PINS) $(AF_FILE) $(PREFIX_FILE) | $(HEADER_BUILD)
 	$(ECHO) "Create $@"
 	$(Q)$(PYTHON) $(MAKE_PINS) --board-csv $(BOARD_PINS) --af-csv $(AF_FILE) --prefix $(PREFIX_FILE) \
 	    --output-source $(GEN_PINS_SRC) --output-header $(GEN_PINS_HDR) --output-af-const $(GEN_PINS_AF_CONST)