Document that LIC_FILES_CHKSUM is mandatory, when gets validated, and
improve some wording.
(From yocto-docs rev: ca878d7b0c84812ef29bbaf5894c96dd89500ec9)
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>