ESP32: allow external USER_C_MODULES from command line (lvgl#100)

Author: shlomozippel

ports/esp32/esp32_common.cmake

@@ -11,7 +11,7 @@ endif()
 # Include core source components.
 include(${MICROPY_DIR}/py/py.cmake)
 
-set(USER_C_MODULES "${MICROPY_DIR}/user_modules/lv_binding_micropython/micropython.cmake")
+list(APPEND USER_C_MODULES "${MICROPY_DIR}/user_modules/lv_binding_micropython/micropython.cmake")
 
 # CMAKE_BUILD_EARLY_EXPANSION is set during the component-discovery phase of
 # `idf.py build`, so none of the extmod/usermod (and in reality, most of the