Added sane defaults for CFLAGS

CFLAGS and friends are currently unset by default.

Since this is designed to run on the Pi, it'd probably be a good idea to add a set of pi-centric CFLAGS. This enables the use of the fpu, as well as specifying a target architecture.

Notably absent are either mtune/mcpu. [Limited reports](http://www.raspberrypi.org/phpBB3/viewtopic.php?f=54&t=17983) say that those are slower. These also seem to be both safe, and provide [good benchmarks.](http://elinux.org/RPi_Performance)

They are still overridable, if the user wants to use softfp or something else insanely dumb like that.