Register automatic git submodules handling

[andrewleech]

Summary

Adds automatic submodule initialization for lvgl and pycparser to both Make and CMake build systems. This removes the manual step of running git submodule update --init before building. Both implementations hook into MicroPython's existing submodules target - Make appends to the target directly, while CMake registers via the ECHO_SUBMODULES mechanism. Also adds validation in CMake to catch missing submodules at configuration time with a helpful error message.

---

Summary by cubic

Automatically initializes lvgl and pycparser submodules in both Make and CMake, removing the manual git submodule step. Adds an early CMake check that fails with a clear message if lvgl isn’t initialized.

• New Features
  • Make: hooks into MicroPython’s submodules target to sync and init lvgl and pycparser.
  • CMake: registers both via ECHO_SUBMODULES and errors early if lvgl is missing (suggests running make BOARD=${MICROPY_BOARD} submodules).

^{Written for commit 8fdb110. Summary will update automatically on new commits.}