esp32/main: Allow MICROPY_DIR to be overridden.

This is necessary when building a custom out-of-tree board.
Signed-off-by: NDamien George <damien@micropython.org>

ports/esp32/main/CMakeLists.txt

 # Set location of base MicroPython directory.
-get_filename_component(MICROPY_DIR ${PROJECT_DIR}/../.. ABSOLUTE)
+if(NOT MICROPY_DIR)
+    get_filename_component(MICROPY_DIR ${PROJECT_DIR}/../.. ABSOLUTE)
+endif()
 
 # Include core source components.
 include(${MICROPY_DIR}/py/py.cmake)