image: Set COREBASE as the git directory for timestamp

image: Set COREBASE as the git directory for timestamp

When REPRODUCIBLE_TIMESTAMP_ROOTFS is unset and we want to parse one
from git, use COREBASE as the base for the git command so we have a
known repository which we're using. Without this the build may fail
if the current directory is not part of a git repository.

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>