emulator: use ext4 instead of yaffs

Squashed commit of the following:

commit e4926d25008a2293fcab41929799bdadfd413535
Author: Nick Kralevich <nnk@google.com>
Date:   Fri Jul 12 20:28:59 2013 -0700

    emulator: increase system partition to 550M

    Yes, 400M isn't enough. I can't believe it either.

    Change-Id: I7d3857f04bfda472af6a281017204672bb9c9940

commit 4b90ddf58fab3cf1164ef272ff696efa33d349cc
Author: Nick Kralevich <nnk@google.com>
Date:   Fri Jul 12 13:44:48 2013 -0700

    emulator: increase system partition size to 400M

    300M is too small.

    Change-Id: Icbe4e76d6e5c8a50cc6e6d74e324335bc66b8466

commit 85a79283c0b8426cbd0350153c43484733ecef5c
Author: Nick Kralevich <nnk@google.com>
Date:   Tue Jul 9 18:04:51 2013 -0700

    use ext4 instead of yaffs for the emulator

    Bug: https://code.google.com/p/android/issues/detail?id=38561
    Change-Id: I7c605457b485d169654263caf55eb7d5a9a60c2e

Change-Id: I3baedb5ecb0dd63c71ae36554173b3ebf5c0d3b0