x86: Don't continue booting if we can't load the specified initrd

If we've determined we can't do what the user asked, trying to do
something else isn't going to make the user's life better.

Without this the screen scrolls a bit and then you get a panic
anyway, and it's nice not to have so much scroll after the real
problem in bug reports.

Link: http://lkml.kernel.org/r/1337190206-12121-1-git-send-email-pjones@redhat.com
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>