toaster: use git reset --hard instead of rebase

toaster: use git reset --hard instead of rebase

Replaced 'git checkout <ref> && git rebase' with 'git reset --hard' as
with git checkout repository ends up with detached HEAD. Rebase makes
things even worse as it can cause conflicts. git reset --hard resets
repository to the required state in a most straightforward and
reliable way.

[YOCTO #7505]

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: brian avery <avery.brian@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>