lisa._kmod: KmodSrc: Separate checksum and checksum_sources

FEATURE

Add a 'checksum_sources' property to KmodSrc.
'checksum' will contain the checksum of all the file names along with
their contents.
'checksum_sources' will only contain the checksum of the actual sources,
excluding the file names and the Makefile.
Extract the common code into a private method _checksum that can
compute both variants depending on the arguments.

Signed-off-by: Kajetan Puchalski <kajetan.puchalski@arm.com>