13.12: update LSK git tag

Due to ARMv8 re-spin, the tag for the LSK kernel is "lsk-vexpress-13.12"
for the 32 bit release.   The tag is still "lsk-13.12" for the formal
LSK release for December, however, that isn't the code used by the 32
bit releases.

Signed-off-by: Ryan Harkin <ryan.harkin@linaro.org>