build: update eat() command with gapps and path fixes

Two changes with the behavior:

  1) Push zip to /data/media/0 in recovery

    In the case when eat was run and the device was not in recovery, it
    would push to /data/media/0/. When the device is in recovery, /sdcard/
    is symlinked to /data/media/, so the file is never found on reboot.

    With this patch, it will push it and flash it to the same location,
    /data/media/0/, where it will be consistent.

  2) Support environment variable $EAT_GAPPS_PATH

    If it is set, then eat will flash the gapps specified right after
    flashing the ROM zip.

    $EAT_GAPPS_PATH should be a path relative to /data/media/0/ and it
    should already be on the device when the command is run. Since this
    file is not going to change often, there's no use in pushing it every
    time.

Change-Id: I9eac3f317e0142d4b840a827f3dc24502a971f13
Signed-off-by: Roman Birg <romanbirg@gmail.com>