Bug 830890 - Make mksh be the default shell in the emulator rather than ash.

ash in the android tree has no [ or test which makes it essentially useless for
non-trivial scripts (like b2g-ps or even run-gdb.sh)
Merge pull request #16 from dhylands/dhylands-830890

Bug 830890 - Make mksh be the default shell in the emulator rather than ash. r=mwu