kernel: update deprecated license information

Update the deprecated license information from GPL-2.0 to GPL-2.0-only
as written in the COPYING file of the linux source tree.

Also add the 'COPYING' file to the PKG_LICENSE_FILES variable.

Signed-off-by: Florian Eckert <[email protected]>

package/kernel/linux/Makefile

@@ -16,8 +16,8 @@ SUBTARGETS = $(sort $(filter-out feeds,$(notdir $(wildcard $(TOPDIR)/target/linu
 SUBTARGET_MODULES = $(foreach t,$(SUBTARGETS),$(firstword $(wildcard $(TOPDIR)/target/linux/feeds/$(t)/modules.mk $(TOPDIR)/target/linux/$(t)/modules.mk)))
 SCAN_DEPS=modules/*.mk $(SUBTARGET_MODULES) $(TOPDIR)/include/netfilter.mk
 
-PKG_LICENSE:=GPL-2.0
-PKG_LICENSE_FILES:=
+PKG_LICENSE:=GPL-2.0-only
+PKG_LICENSE_FILES:=COPYING
 
 export SHELL:=/bin/sh
 .ONESHELL: