Derald D. Woods authored Jan 20, 2018 and Tom Rini committed Jan 28, 2018

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>