envsetup: Add pushboot function

This function lets you push a file from your current OUT dir
for your current device. It root/remount the device, push the
file, and reboots it. Example: pushboot system/app/SystemUI.apk

Change-Id: I8a404323a4091e8502fbdd24198c66f1ac598192
Merge "envsetup: Add pushboot function" into android-4.4