igep00x0: UBIize

igep00x0: UBIize

Convert IGEP board to use UBI volumes for U-Boot, its environment and
kernel. With exception of first four sectors read by SoC boot
ROM whole (One)NAND is UBI managed.
Also merge NAND and OneNAND defconfigs as now one binary can serve
both flashes.
As code is too big now, drop CONFIG_SPL_EXT_SUPPORT to make it fit.

Signed-off-by: Ladislav Michl <ladis@linux-mips.org>
Reviewed-by: Heiko Schocher <hs@denx.de>