Add MIPS support

Tested on:
 - Ainol Novo 7 Paladin (JZ4770 MIPS SoC) with ICS 4.0.3 and CWM
 - Nexus 5 (ARM) with KK 4.4 and TWRP

Not tested on x86.

I checked to make sure "uname -m" is standard in busybox (not a feature
flag).  "uname" doesn't seem to exist at all in AOSP, so without busybox,
the original script probably wouldn't have detected the architecture
correctly.