Incorrect checkout of git submodules

When checking out git packages, buildroot doesn't seem to track the revisions
correctly of any submodules referenced by that project. As a result, the
submodule stays at whatever revision was referenced by the head of the master
branch. Running a 'git submodule update' after the checkout fixes this problem.

Signed-off-by: Owen Kirby <osk@exegin.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@38359 3c298f89-4303-0410-b956-a3cf2f4a3e73