license.bbclass: Don't copy again LICENSE already handled as no-generic

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.

[YOCTO #10325]

[1] http://lists.openembedded.org/pipermail/openembedded-core/2015-April/104222.html

(From OE-Core rev: 95b9e2cd26c7cae265ff52af90480b75251f00e5)

Signed-off-by: Aníbal Limón <anibal.limon@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>