bitbake.conf: add TMPDIR to GIT_CEILING_DIRECTORIES

bitbake.conf: add TMPDIR to GIT_CEILING_DIRECTORIES

We export GIT_CEILING_DIRECTORIES=WORKDIR to ensure that git calls
inside the builds don't find oe-core when they're meant to be looking
for the git repository of the source code.

However, this breaks for recipes that use work-shared (such as llvm), as
their working directory is outside of WORKDIR.

Solve this by adding TMPDIR to the list as a final catch, but keeping
WORKDIR first so that git will stop sooner in the general case.

This solves reproduciblity problems in LLVM, where for example lld's
version string would contain the URL and commit hash of the poky repo
being built.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>