Fix 'make submodules' when building out-of-tree - #14344

When MicroPython is used as a submodule and built using make from the containing project, e.g. following the instructions for the embed port, make submodules fails because it goes looking for the sub-sub-module paths in the outer repository instead of in the micropython repository.

This is fixed by the attached commit: invoke git inside the micropython submodule instead of at the outer project level.

(I initially thought that this would affect not just the embed port but also external board definitions, but it turns out it does not, because in the external-board-definition process MicroPython is build by a sub-make that operates inside the MicroPython submodule.)

Steps to reproduce

git init mpembedder
cd mpembedder
git submodule add https://github.com/micropython/micropython.git micropython
cp micropython/examples/embedding/* .
perl -pi -e 's|MICROPYTHON_TOP = ../..|MICROPYTHON_TOP = micropython|' micropython_embed.mk
echo 'require("time")' > manifest.py
make -f micropython_embed.mk FROZEN_MANIFEST=manifest.py submodules

Actual result

Use make V=1 or set BUILD_VERBOSE in your environment to increase build verbosity.
Updating submodules: lib/micropython-lib
error: pathspec 'micropython/lib/micropython-lib' did not match any file(s) known to git
make: *** [submodules] Error 1

Expected result

Use make V=1 or set BUILD_VERBOSE in your environment to increase build verbosity.
Updating submodules: lib/micropython-lib
Submodule 'lib/micropython-lib' (https://github.com/micropython/micropython-lib.git) registered for path 'lib/micropython-lib'
Cloning into '/Users/cwalther/mpembedder/micropython/lib/micropython-lib'...
Submodule path 'lib/micropython-lib': checked out 'ddb1a279578bfff8c1b18aff3baa668620684f64'