kernel-yocto: don't require meta branch for custom repos

kernel-yocto: don't require meta branch for custom repos

The commit a9f11849 [linux-yocto: explicitly export KMETA to scripts]
allows the meta branch name to be changed by exporting it to all
phases of the build.

But if a custom kernel without a meta branch is built, we end up
passing an empty string to the creation scripts, which breaks the
build since input is expected.

Inhibiting the export of KMETA to the creation scripts when empty
fixes the problem.

(From OE-Core rev: 61162fbe01993659301fe2e821bf9c3e801206d8)

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