mips: revert commit b332d24a

Now that this code path is not triggered anymore during the tests,
revert commit b332d24a. Booting a MIPS
target without kernel nor bios doesn't really make sense. At the same
time replace fprintf(stderr, ...) by error_report().

Reviewed-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>