update cmake vars which come from environment with FORCE so they can be updated (#1405)

Author: kilograham

src/board_setup.cmake

@@ -10,7 +10,7 @@ else()
         message("PICO target board is ${PICO_BOARD}.")
     endif()
 endif()
-set(PICO_BOARD ${PICO_BOARD} CACHE STRING "PICO target board (e.g. pico)")
+set(PICO_BOARD ${PICO_BOARD} CACHE STRING "PICO target board (e.g. pico)" FORCE)
 
 # PICO_CMAKE_CONFIG: PICO_BOARD_CMAKE_DIRS, Directories to look for <PICO_BOARD>.cmake in. This is overridable from the user environment, type=list, default="", group=build
 if (DEFINED ENV{PICO_BOARD_CMAKE_DIRS})

src/boards/generic_board.cmake

@@ -5,7 +5,7 @@ if (DEFINED ENV{PICO_BOARD_HEADER_DIRS})
     set(PICO_BOARD_HEADER_DIRS $ENV{PICO_BOARD_HEADER_DIRS})
     message("Using PICO_BOARD_HEADER_DIRS from environment ('${PICO_BOARD_HEADER_DIRS}')")
 endif()
-set(PICO_BOARD_HEADER_DIRS ${PICO_BOARD_HEADER_DIRS} CACHE STRING "PICO board header directories")
+set(PICO_BOARD_HEADER_DIRS ${PICO_BOARD_HEADER_DIRS} CACHE STRING "PICO board header directories" FORCE)
 
 list(APPEND PICO_BOARD_HEADER_DIRS ${CMAKE_CURRENT_LIST_DIR}/include/boards)
 pico_find_in_paths(PICO_BOARD_HEADER_FILE PICO_BOARD_HEADER_DIRS ${PICO_BOARD}.h)