Rewrite the license checksum generation and loading of CSV files to be
clearer.
This also expands the scan of COMMON_LICENSE_DIR to include LICENSE_PATH,
which can be extended by layers to provide more license texts.
(From OE-Core rev: 417240ba7a9b3985530988940a222b079b503b64)
Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>