kernel: Make sure source tree is totally clean

Left over source tree generated files for picoimx will cause builds to
fail. To handle this, force a "make mrproper" in kernel source trees
before a build to make sure that all generated files have been removed.

In theory, this can be removed once picoimx has been cleaned up.

Change-Id: Iaa8980e2a3add58c179c178a8ef77665b703042a
1 file changed