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>

libs/libmodbus/Makefile

@@ -9,7 +9,7 @@ include $(TOPDIR)/rules.mk
 
 PKG_NAME:=libmodbus
 PKG_VERSION:=3.1.4
-PKG_RELEASE:=2
+PKG_RELEASE:=3
 
 PKG_SOURCE:=$(PKG_NAME)-$(PKG_VERSION).tar.gz
 PKG_SOURCE_URL:=http://libmodbus.org/releases
@@ -17,7 +17,7 @@ PKG_HASH:=c8c862b0e9a7ba699a49bc98f62bdffdfafd53a5716c0e162696b4bf108d3637
 
 PKG_MAINTAINER:=Michael Heimpold <mhei@heimpold.de>
 
-PKG_LICENSE:=GPL-3.0+ LGPL-2.1+
+PKG_LICENSE:=GPL-3.0-or-later LGPL-2.1-or-later
 PKG_LICENSE_FILES:=COPYING COPYING.LESSER
 
 PKG_FIXUP:=autoreconf