cmd: mvebu: Hide bubt specific options when bubt is disabled

CONFIG_MVEBU_NAND_BOOT, CONFIG_MVEBU_SPI_BOOT, CONFIG_MVEBU_MMC_BOOT and
CONFIG_MVEBU_UBOOT_DFLT_NAME are unused when CONFIG_CMD_MVEBU_BUBT is not
enabled. So hide them.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>