releasetools: only allow yaffs to have no userdata image size (fix build)

In the past, there was an exception for ext-base fs types to
deal with the lack of image size.
Back then it was only yaffs and ext*.
So now we explicitely only allow yaffs to have no userdata image size.

Change-Id: Ie354ee6222a58228dbcce2c6934971a0737422af
Signed-off-by: JP Abgrall <jpa@google.com>