rp2/Makefile: Add support for BOARD_VARIANTS.

Following stm32.  This allows a single board definition to define variants
of its configuration.

ports/rp2/Makefile

@@ -22,6 +22,10 @@ ifeq ($(DEBUG),1)
 CMAKE_ARGS += -DCMAKE_BUILD_TYPE=Debug
 endif
 
+ifdef BOARD_VARIANT
+CMAKE_ARGS += -DBOARD_VARIANT=${BOARD_VARIANT}
+endif
+
 HELP_BUILD_ERROR ?= "See \033[1;31mhttps://github.com/micropython/micropython/wiki/Build-Troubleshooting\033[0m"
 
 all:
@@ -38,3 +42,8 @@ submodules:
 	GIT_SUBMODULES=$$(cmake -B $(BUILD)/submodules -DECHO_SUBMODULES=1 ${CMAKE_ARGS} -S . 2>&1 | \
 	                  grep '^GIT_SUBMODULES=' | cut -d= -f2); \
 	$(MAKE) -f ../../py/mkrules.mk GIT_SUBMODULES="$${GIT_SUBMODULES}" submodules
+
+query-variants:
+	@BOARD_VARIANTS=$$(cmake -B $(BUILD)/variants -DECHO_BOARD_VARIANTS=1 ${CMAKE_ARGS} -S . 2>&1 | \
+	                  grep '^BOARD_VARIANTS=' | cut -d= -f2); \
+	echo "VARIANTS: $${BOARD_VARIANTS}"

py/mkrules.cmake

@@ -223,3 +223,9 @@ if(ECHO_SUBMODULES)
     execute_process(COMMAND ${CMAKE_COMMAND} -E echo "GIT_SUBMODULES=${GIT_SUBMODULES}")
     message(FATAL_ERROR "Done")
 endif()
+
+# Display BOARD_VARIANTS
+if(ECHO_BOARD_VARIANTS)
+    execute_process(COMMAND ${CMAKE_COMMAND} -E echo "BOARD_VARIANTS=${BOARD_VARIANTS}")
+    message(FATAL_ERROR "Done")
+endif()