esp32/Makefile: Allow specifying BOARD_DIR on make command line.

This matches the behavior of the makefile ports but implemented for CMake,
making it easy to specify custom board definitions.

This work was funded through GitHub Sponsors.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

ports/esp32/Makefile

@@ -2,8 +2,20 @@
 #
 # This is a simple, convenience wrapper around idf.py (which uses cmake).
 
-# Select the board to build for, defaulting to GENERIC.
+# 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 $(BOARD_DIR:/=))
+else
+# If not given on the command line, then default to GENERIC.
 BOARD ?= GENERIC
+BOARD_DIR ?= boards/$(BOARD)
+endif
+
+ifeq ($(wildcard $(BOARD_DIR)/.),)
+$(error Invalid BOARD specified: $(BOARD_DIR))
+endif
 
 # If the build directory is not given, make it reflect the board name.
 BUILD ?= build-$(BOARD)
@@ -28,7 +40,7 @@ ifdef USER_C_MODULES
 	CMAKE_ARGS += -DUSER_C_MODULES=${USER_C_MODULES}
 endif
 
-IDFPY_FLAGS += -D MICROPY_BOARD=$(BOARD) -B $(BUILD) $(CMAKE_ARGS)
+IDFPY_FLAGS += -D MICROPY_BOARD=$(BOARD) -D MICROPY_BOARD_DIR=$(abspath $(BOARD_DIR)) -B $(BUILD) $(CMAKE_ARGS)
 
 ifdef FROZEN_MANIFEST
        IDFPY_FLAGS += -D MICROPY_FROZEN_MANIFEST=$(FROZEN_MANIFEST)