[LinuxBIOS] [PATCH] Allow setting ROM_SIZE for qemu target

[Ed Swierk]

Currently the qemu target is configured to always use a fallback
image, and the size of the image is hardcoded to 128KB.

The attached patch allows changing the image size by setting ROM_SIZE
in the target Config.lb.

It removes all the fallback-related stuff from the qemu mainboard
Config.lb, since it wasn't doing anything useful and I couldn't think
of a reason to keep it.

--Ed