kernel-yocto: fix branch validation for AUTOREV, non machine_meta kernels

kernel-yocto: fix branch validation for AUTOREV, non machine_meta kernels

The simplication of do_validate_branches missed a case where a custom
kernel can supply SRCREV="${AUTOREV}", and not use SRCREV_machine at all.

In this case, we will incorrectly try and test the tree for a non-existent
commit, and break the build.

By simplying the condition of the check to look for an empty SRCREV_machine,
we can skip manipulating the tree and testing for a SRCREV.

Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>