yocto-bsp: add basic git connectivity check

yocto-bsp: add basic git connectivity check

yocto-bsp create does a 'git ls-remote
git://git.yoctoproject.org/linux-yocto-3.4.git *heads*' to get the set
of existing branches from the kernel repo.

If the user isn't connected to the network, or if git isn't configured
sanely, yocto-bsp fails with an ugly Python backtrace.

We should try to avoid this by doing a basic sanity check for those
things before actually running the command.

The sanity check can be avoided by specifying -s on the yocto-bsp
command-line:

 $ yocto-bsp create -s test qemu

Fixes [YOCTO #3279]

(From meta-yocto rev: 496e76f9bed2ed5a04ef757724d2e63d05c7a601)

Signed-off-by: Tom Zanussi <tom.zanussi@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>