Manifest files containing the same duplicated prefix are wasteful on space
and ultimately this costs build time. Add support for manifest files with
common prefixes removed and use the prefix if the path isn't absolute.
(From OE-Core rev: 71e8c98238d627f9665faee9d7c226b9c07bd256)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>