The difference between bb-layers and bb-layers-relative is unclear as both
are relative paths. Rename one to "file-relative" which makes it clear it
is relative to the current file, without becomming a long name.
https://lists.openembedded.org/g/bitbake-devel/message/18296
Based on a patch from Alexander Kanavin <alex@linutronix.de> but
with different naming.
(Bitbake rev: dcb17758b99767ab6da4172cf60eabc9269082dd)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>