mdk3: fix compile on platforms with VFP

mdk3: fix compile on platforms with VFP

buildbot reported error on platforms with VFP (sunxi,imx6,omap):
mdk3 uses VFP register arguments, /tmp/ccBN2WMD.o does not
mdk3 uses VFP register arguments, osdep/libosdep.a(osdep.o) does not
mdk3 uses VFP register arguments, osdep/libosdep.a(network.o) does not
mdk3 uses VFP register arguments, osdep/libosdep.a(linux.o) does not
mdk3 uses VFP register arguments, osdep/libosdep.a(radiotap-parser.o) does not
mdk3 uses VFP register arguments, osdep/libosdep.a(common.o) does not
with failed to merge target specific data of file ...

CFLAGS were overwritten so append them instead

Signed-off-by: Dirk Neukirchen <dirkneukirchen@web.de>
Signed-off-by: Florian Fainelli <florian@openwrt.org>
git-svn-id: svn://svn.openwrt.org/openwrt/packages@40789 3c298f89-4303-0410-b956-a3cf2f4a3e73