treewide: Change .*GPL.*+ licenses to SPDX compatible identifier

The CONTRIBUTING.md requests an (or multiple) SPDX identifier for GPL
licenses. But a lot of packages did use a different, non-SPDX style with a
"+" at the end instead of "-or-later".

Signed-off-by: Sven Eckelmann <sven@narfation.org>

net/siit/Makefile

@@ -10,8 +10,8 @@ include $(INCLUDE_DIR)/kernel.mk
 
 PKG_NAME:=siit
 PKG_VERSION:=1.2
-PKG_RELEASE:=2
-PKG_LICENSE:=GPLv2+
+PKG_RELEASE:=3
+PKG_LICENSE:=GPL-2.0-or-later
 
 include $(INCLUDE_DIR)/package.mk