only fetch if .gitmodules exists
in some cases, lbmk is compiling source trees that
use submodules, without having downloaded them first.
in all cases, those submodules are either optional,
or the build system auto-fetches them (or if it can,
we sometimes disable it as with grub and gnulib).
this is a nice fallback behaviour, for situations where
we forget to put submodules as dependencies under
config/git (and disable submodules in the given project).
with this change, release archives are guaranteed to
be complete, sans crossgcc downloads in coreboot; this
will be handled in a follow-up commit.
Signed-off-by: Leah Rowe <leah@libreboot.org>
54eb347a30