bitbake: bitbake/fetch: Add git submodules fetcher

bitbake: bitbake/fetch: Add git submodules fetcher

This adds very basic git submodule support to the fetcher. It can be
used by replacing a git:// url prefix with a gitsm:// prefix, otherwise
behaviour is the same as the git fetcher. Whilst this code should be
functional, its not as efficient as the usual git fetcher due to the
need to checkout the tree to fetch/update the submodule information. git
doesn't support submodule operations on the bare clones the standard git
fetcher uses which is also problematic.

This code does however give a starting point to people wanting to use
submodules.

(Bitbake rev: 25e0b0bc50114f1fbf955de23cc0c96f5f7a41e3)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>