kernel.bbclass: fix SOURCE_DATE_EPOCH for non-git kernel builds
The source directory is not always a git repository, so when querying
git for data to set SOURCE_DATE_EPOCH, specify ${S}/.git as the git
directory to prevent retrieving incorrect data from any parent directory.
Fixes the following errors with the prior logic when building a kernel
that is not obtained from a git repository:
1. With TMPDIR set to a directory outside any git repository on a
mounted filesystem, reproducible builds fail in do_compile with this git
error:
fatal: not a git repository (or any parent up to mount point <abspath>)
Stopping at filesystem boundary (GIT_DISCOVERY_ACROSS_FILESYSTEM not set).
aborting before the error handling logic.
2. With TMPDIR located within a subdirectory of a git repository, the
SOURCE_DATE_EPOCH timestamp would be that of said repository rather than
that of the kernel.
Signed-off-by: Christopher Clark <christopher.w.clark@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
