Also use mkrules.mk's submodule target rather than duplicating the call to
`submodule sync`.
Until we can find a way to use idf.py/cmake to discover submodules we have
no way to discover optional or board-specific submodules so need to err on
the side of including everything.
Signed-off-by: Jim Mussared <jim.mussared@gmail.com>
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