Add check_license_format function that shows warning if LICENSE don't have
valid operators and also if have space separated entries without operator,
add check_license_format validation into base class.
[YOCTO #6758]
(From OE-Core rev: 346a023a42f127881476e760e8fa4e04303849b9)
Signed-off-by: Aníbal Limón <anibal.limon@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>