rp2/CMakeLists: Allow relative MICROPY_BOARD_DIR when invoking cmake.

Convert to an absolute path to always reliably locate manifest.py.  This is
already done in Makefile, but is also needed in CMakeLists.txt if cmake is
invoked directly.

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

ports/rp2/CMakeLists.txt

@@ -31,6 +31,7 @@ endif()
 if(NOT MICROPY_BOARD_DIR)
     set(MICROPY_BOARD_DIR ${MICROPY_PORT_DIR}/boards/${MICROPY_BOARD})
 endif()
+get_filename_component(MICROPY_BOARD_DIR ${MICROPY_BOARD_DIR} ABSOLUTE)
 if(NOT EXISTS ${MICROPY_BOARD_DIR}/mpconfigboard.cmake)
     message(FATAL_ERROR "Invalid MICROPY_BOARD specified: ${MICROPY_BOARD}")
 endif()