Fix the build when srcdir == objdir

You would only see this error on a fresh clone when srcdir == objdir.  configure
will fail because roms/pcbios doesn't exist.

git submodule integration doesn't cleanup very well when switching between
branches so you'll get an roms/pcbios directory from normal operations if you
switch between old branches.

Thanks to a mistake in configure, if you build outside of srcdir, you'll also
get a valid roms/pcbios.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
