lib/buildcfg: Add is_bitbake_in_separate_repo()

This would help various tooling that is being developed for setting up layers
and builds, or creating configurations for those tasks out of pre-existing
yocto setups.

Suggested-by: Alexander Kanavin <alex.kanavin@gmail.com>
Signed-off-by: Robert Yang <liezhi.yang@windriver.com>