license.bbclass: allow copying license not in common licenses
Some package like linux-firmware has many licenses that aren't in any
way common, and new ones will be added from time to time, in order to
avoid adding bunch of such common license files that are only applicable
to a specific package, NO_GENERIC_LICENSE is added to allow copying license
not in common licenses, it should be used in the recipe as:
NO_GENERIC_LICENSE[<license_name>] = "<license file in fetched source>"
e.g.
NO_GENERIC_LICENSE[Firmware-Abilis] = "LICENCE.Abilis.txt"
