kernel-yocto: Simplify no git repo case in do_kernel_checkout
If the kernel sources are not fetched via git, a local git repository is
created in do_kernel_checkout. In this case we know that there will be
no remote branches and we will already be on the correct branch (since
only one branch will exist). So we can simplify things by skipping these
steps.
This also removes the assumption that the default git branch name will
be "master". Prior to this change, the final git checkout command in
do_kernel_checkout could fail if a local git repo was created and the
user had changed init.defaultBranch in their gitconfig.
Signed-off-by: Paul Barker <paul@pbarker.dev> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
