linux: update license tag to use correct SPDX tag

Use SPDX tag.

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

package/kernel/linux/Makefile

@@ -14,7 +14,7 @@ PKG_FLAGS:=hold
 PKG_BUILD_DIR:=$(KERNEL_BUILD_DIR)/packages
 SCAN_DEPS=modules/*.mk $(TOPDIR)/target/linux/*/modules.mk $(TOPDIR)/include/netfilter.mk
 
-PKG_LICENSE:=GPLv2
+PKG_LICENSE:=GPL-2.0
 PKG_LICENSE_FILES:=
 
 export SHELL:=/bin/sh