kernel-yocto: ensure sccs variable is set when using KBUILD_DEFCONFIG

kernel-yocto: ensure sccs variable is set when using KBUILD_DEFCONFIG

When using KBUILD_DEFCONFIG, $sccs should be set to the $WORKDIR/defconfig
regardless if it compares or is copied. Otherwise $sccs is not set and the
defconfig is not found correctly.

Part of
[YOCTO #12162]

(From OE-Core rev: b63cc051f1eb58c768f49db2c04843336e62d3df)

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