ports: Make BOARD default from BOARD_DIR in Makefile's.

This allows:

    $ make BOARD_DIR=path/to/board

to infer BOARD=board, rather than the previous behavior that required
additionally setting BOARD explicitly.

Also makes the same change for VARIANT_DIR -> VARIANT on Unix.

This work was funded through GitHub Sponsors.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>
Signed-off-by: Damien George <damien@micropython.org>

ports/samd/Makefile

@@ -1,14 +1,23 @@
+# Select the board to build for:
+ifdef BOARD_DIR
+# Custom board path - remove trailing slash and get the final component of
+# the path as the board name.
+BOARD ?= $(notdir $(BOARD_DIR:/=))
+else
+# If not given on the command line, then default to ADAFRUIT_ITSYBITSY_M4_EXPRESS.
 BOARD ?= ADAFRUIT_ITSYBITSY_M4_EXPRESS
 BOARD_DIR ?= boards/$(BOARD)
-BUILD ?= build-$(BOARD)
-
-CROSS_COMPILE ?= arm-none-eabi-
-UF2CONV ?= $(TOP)/tools/uf2conv.py
+endif
 
 ifeq ($(wildcard $(BOARD_DIR)/.),)
 $(error Invalid BOARD specified: $(BOARD_DIR))
 endif
 
+BUILD ?= build-$(BOARD)
+
+CROSS_COMPILE ?= arm-none-eabi-
+UF2CONV ?= $(TOP)/tools/uf2conv.py
+
 MCU_SERIES_LOWER = $(shell echo $(MCU_SERIES) | tr '[:upper:]' '[:lower:]')
 
 include ../../py/mkenv.mk