Stop ignoring tmp-eglibc and the cache dir.

eglibc and glibc have finally merged back together so the tmp dir
is now tmp-glibc anyways. Still we don't want this ignored because
it's super convenient to be able to reset the whole git dir to clean
this stuff out.
Use bash for build.sh so we can use PIPESTATUS to get the exit code from
the bitbake invocation.