[B2G] add device pull script

[gasolin]

from
https://developer.mozilla.org/en-US/docs/Mozilla/Firefox_OS/Firefox_OS_build_prerequisites#Backup_the_phone_system_partition
and
http://shizen008.wordpress.com/2013/06/12/how-to-build-an-ubuntu-vm-that-compiles-b2g/#comment-100

a) rm -rf backup-
b) go to the folder that contains extract-device.sh ie devices/<whatever, whatever>//extract-device.sh
c) run adb root
d) run adb remount
e) sudo ./extract-device.sh