license.bbclass: Don't copy again LICENSE already handled as no-generic
The NO_GENERIC_LICENSE mapping was added [1] to enable copy LICENSES
from upstream source code into recipe licenses, previously that only
common-licenses was processed.
This result on copy twice the NO_GENERIC_LICENSE specified because there
is a mapping between license in LIC_CHKSUM and NO_GENERIC_LICENSE.
In order to avoid double copy one as generic_ and other as LICENSE. keep
track of licenses already copied.
For linux-firmware the result will be only generic_ licenses into
common-licenses.
