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: NDerald D. Woods <woods.technical@gmail.com>

include/config_distro_bootcmd.h

@@ -71,9 +71,14 @@
 #ifdef CONFIG_CMD_UBIFS
 #define BOOTENV_SHARED_UBIFS \
 	"ubifs_boot=" \
-		"if ubi part UBI && ubifsmount ubi${devnum}:boot; then "  \
+		"env exists bootubipart || " \
+			"env set bootubipart UBI; " \
+		"env exists bootubivol || " \
+			"env set bootubivol boot; " \
+		"if ubi part ${bootubipart} && " \
+			"ubifsmount ubi${devnum}:${bootubivol}; " \
+		"then " \
 			"setenv devtype ubi; " \
-			"setenv bootpart 0; "                             \
 			"run scan_dev_for_boot; " \
 		"fi\0"
 #define BOOTENV_DEV_UBIFS	BOOTENV_DEV_BLKDEV