MIPS: Sibyte: Remove standalone kernel support

CFE is the only supported and used bootloader on the SiByte boards,
the standalone kernel support has been never used outside Broadcom.
Remove it and make the kernel use CFE by default.

Signed-off-by: Imre Kaloz <kaloz@openwrt.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>