distro bootcmd: Allow board defined UBI partition and volume names

This commit allows overriding the default assumption that the boot UBI
MTD partition is named 'UBI' and the UBI volume is 'boot'. A board
desiring to use a legacy or alternative NAND layout can now define the
following two extra environment variables:

	bootubipart=<some_ubi_partition_name>
	bootubivol=<some_ubi_volume_name>

EXAMPLE:

[include/configs/some_board.h]
---8<-------------------------------------------------------------------
[...]
	#include <config_distro_defaults.h>

	#define MEM_LAYOUT_ENV_SETTINGS \
		DEFAULT_LINUX_BOOT_ENV

	#define BOOT_TARGET_DEVICES(func) \
		func(UBIFS, ubifs, 0)

	#include <config_distro_bootcmd.h>
[...]
	#define CONFIG_EXTRA_ENV_SETTINGS \
		MEM_LAYOUT_ENV_SETTINGS \
		"bootubivol=rootfs\0" \
		"bootubipart=rootfs\0" \
		BOOTENV
[...]
---8<-------------------------------------------------------------------

Signed-off-by: Derald D. Woods <woods.technical@gmail.com>