README: Update build information

Specify the required SDK API and NDK versions.  Add info on signing the
release build, as build.xml is hardcoded to use Superuser-release.apk only.
reboot: Build utility from source

This was based on disassembling the HTC ARM reboot binary currently
shipped with Superuser.  It attempts to mimic the behavior of the
original program.

Future work may involve cleaning it up, removing the HTC-specific
features, etc.
MainActivity: Split getArch() into a common function
Use new assets/<ARCH>/reboot binary

Now that we are building the reboot binary from source, we'll need to
retrieve it from the proper subdirectory according to the device's CPU
architecture.
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.