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>
(cherry-picked from bbb1ea7345)

net/pen/Makefile

@@ -9,13 +9,13 @@ include $(TOPDIR)/rules.mk
 
 PKG_NAME:=pen
 PKG_VERSION:=0.34.1
-PKG_RELEASE:=2
+PKG_RELEASE:=3
 
 PKG_SOURCE:=$(PKG_NAME)-$(PKG_VERSION).tar.gz
 PKG_SOURCE_URL:=http://siag.nu/pub/pen/
 PKG_HASH:=2b640795029df9d1672e17202c109cc5d42538f6754a6070dc27da640881e864
 
-PKG_LICENSE:=GPL-2.0+
+PKG_LICENSE:=GPL-2.0-or-later
 PKG_LICENSE_FILES:=COPYING
 PKG_MAINTAINER:=