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.
Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
