kernel-yocto: fix checkout bare-cloned kernel repositories

kernel-yocto: fix checkout bare-cloned kernel repositories

The existing code doesn't tell regular (with .git) and bare cases and
just move the unpacked repo to the place of kernel source. But later
steps will fail on a bare-cloned repo because we can not checkout
directly in a bare cloned repo.

This change performs another clone to fix the issue.

Note: This change doesn't cover the case that S and WORKDIR are same
and the repo is bare cloned.

(From OE-Core rev: ccfa2ee5c4f509de4c18a7054b2a66fc874d5d69)

Signed-off-by: Jianxun Zhang <jianxun.zhang@linux.intel.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>