Add more license tags with SPDX identifiers

Note, that licensing stuff is a nightmare: many packages does not clearly
state their licenses, and often multiple source files are simply copied
together - each with different licensing information in the file headers.

I tried hard to ensure, that the license information extracted into the OpenWRT's
makefiles fit the "spirit" of the packages, e.g. such small packages which
come without a dedicated source archive "inherites" the OpenWRT's own license
in my opinion.

However, I can not garantee that I always picked the correct information
and/or did not miss license information.

Signed-off-by: Michael Heimpold <mhei@heimpold.de>

SVN-Revision: 43155

package/network/utils/iptables/Makefile

@@ -19,6 +19,9 @@ PKG_SOURCE_URL:=http://www.netfilter.org/projects/iptables/files \
 	ftp://ftp.no.netfilter.org/pub/netfilter/iptables/
 PKG_MD5SUM:=536d048c8e8eeebcd9757d0863ebb0c0
 
+PKG_LICENSE:=GPL-2.0+
+PKG_LICENSE_FILES:=COPYING
+
 PKG_FIXUP:=autoreconf
 PKG_INSTALL:=1
 PKG_BUILD_PARALLEL:=1