linux-yocto: make kernel_checkout reentrant

linux-yocto: make kernel_checkout reentrant

The steps in do_kernel_checkout modify the source tree in WORKDIR.
If it is called multiple times, or interrupted, the tree is left
in an inconsistent state.

This change adds protections around branch names, and around the
manipulations of directories to ensure that it is safe to call
at any point.

(From OE-Core rev: f937977f241e786c5a7438449ed4c9da4c55829b)

Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>