allow both endiannesses coexist.
* configure.ac: proparate target_cpu=mipsel rather than resetting to
mips. All conditions adjusted.
* tests/util/grub-shell-tester.in: Remove gratuitious target_cpu
variable.
* util/grub-install.in: Adjust conditions to take renaming into account.
* util/grub-mkimage.c (image_targets): Likewise. New target
mips-qemu_mips-elf for bigendian mips.