buildroot: improve git submodule handling for packages

Move the `--recursive` switch from `git clone` to `git submodule`
so that submodules are cloned for upstream branches where the
PKG_SOURCE_VERSION commit-ish has a different .gitmodules
configuration than the repository default.

This is, for example, required when the master branch for a source
package does not use submodules, but its topic branch for OpenWRT
does.

This changes the buildroot dependency from git-1.6.2 to git 1.7.12.2,
which was released September 2012.

Signed-off-by: Darik Horn <dajhorn@vanadac.com>
Signed-off-by: Karl Palsson <karlp@tweak.net.au>

SVN-Revision: 48830