Working b2g emulator

To get working b2g emulator:
- Use Ubuntu14 VMWare image, 60GB storage, 4GB memory
- Follow https://developer.mozilla.org/en-US/Firefox_OS/Building_and_installing_Firefox_OS
 - Build mozilla-central firefox also on it (see bug 1002545):

# hg clone http://hg.mozilla.org/mozilla-central
# cd mozilla-central

# ./mach bootstrap

# ./mach build
# export XULRUNNER_DIRECTORY=$PWD/obj-x86_64-unknown-linux-gnu/dist

Then go to B2G folder and run ./mach mochitest-remote