pkg-generic: host variant use git submodules if target variant does

pkg-generic: host variant use git submodules if target variant does

When a package has both a target and a host variant, and uses git
submodules, and the host variant is downloaded before the target one, we
end up with the generated archive missing the submodules.

This happens in exactly one package in our tree: c-capnproto.

This issue was not caught before because after a few days, the full
sources are added to sources.buildroot.net. So when the hash check
fails, the full tarball is simply downloaded from there.

Propagate the git submodule setting from the target variant to the host
variant, unless the host variant explicitly opted-out.

Fixes:
    http://autobuild.buildroot.org/results/2de9c6c8ce83569d18cc7140ebc60d6fe1aadcbf/

Reported-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>