When MicroPython is used as a submodule and built from the containing
project, e.g. 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. Fix this by invoking git inside the micropython
submodule.
Signed-off-by: Christian Walther <cwalther@gmx.ch>