- Add patch to inhibit the libm IEEE math switch on musl
 - Add postprocessing for config.sh to the Makefile to fixup
   defines for musl, idea taken
   from http://patchwork.openembedded.org/patch/91707/

This is the least invasive approach I could come up to fix the build
failure asap. Another possibility is maintaining yet another set of
musl specific architecture config files, but I think that this introduces
a lot of maintenance overhead.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>