rp2/CMakeLists.txt: Add MICROPY_DEF_BOARD to compile definitions.

Add MICROPY_DEF_BOARD as per esp32 port, allows board variants to override
the board name with:

    list(APPEND MICROPY_DEF_BOARD
        MICROPY_HW_BOARD_NAME="New Board Name"
    )

Signed-off-by: Phil Howard <phil@gadgetoid.com>

ports/rp2/CMakeLists.txt

@@ -493,6 +493,7 @@ set_source_files_properties(
 )
 
 target_compile_definitions(${MICROPY_TARGET} PRIVATE
+    ${MICROPY_DEF_BOARD}
     FFCONF_H=\"${MICROPY_OOFATFS_DIR}/ffconf.h\"
     LFS1_NO_MALLOC LFS1_NO_DEBUG LFS1_NO_WARN LFS1_NO_ERROR LFS1_NO_ASSERT
     LFS2_NO_MALLOC LFS2_NO_DEBUG LFS2_NO_WARN LFS2_NO_ERROR LFS2_NO_ASSERT