use ext4 instead of yaffs

This is needed to allow the emulator to use ext4 images.

Bug: https://code.google.com/p/android/issues/detail?id=38561
Change-Id: I02fde6382ad67cf7f4f3b695396841f70bb9301d
Signed-off-by: Yen-Chin Lee <coldnew.tw@gmail.com>

Conflicts:
	rootdir/init.rc