ubispl: add support for loading volumes by name

The motivation is to use the UBI atomic volume rename functionality to
allow double copy software updates on UBI. To that end the SPL is
configured to always load the same volume name (e.g. "u-boot"),
whereas a software updater always installs into the secondary volume
"u-boot_r". After successful installation, these two volume names are
switched.

This extension is protected by #ifdefs as it will somewhat slow down
loading of volumes by id. This is because the code needs to disable
the optimization of ignoring all volume ids which are not
to-be-loaded, since these can only be resolved after attaching.

This adds two vtbl related functions from Linux, which are taken from
the same kernel version as the current main U-Boot UBI code (Linux 4.2
64291f7db5bd8).

Signed-off-by: Hamish Guthrie <hamish.guthrie@kistler.com>
Signed-off-by: Markus Klotzbuecher <markus.klotzbuecher@kistler.com>
Reviewed-by: Heiko Schocher <hs@denx.de>
Cc: Kyungmin Park <kmpark@infradead.org>
diff --git a/common/spl/Kconfig b/common/spl/Kconfig
index 31eac5a..716b1ec 100644
--- a/common/spl/Kconfig
+++ b/common/spl/Kconfig
@@ -620,6 +620,13 @@
 	  README.ubispl for more info.
 
 if SPL_UBI
+config SPL_UBI_LOAD_BY_VOLNAME
+	bool "Support loading volumes by name"
+	help
+	  This enables support for loading UBI volumes by name. When this
+	  is set, CONFIG_SPL_UBI_LOAD_MONITOR_VOLNAME can be used to
+	  configure the volume name from which to load U-Boot.
+
 config SPL_UBI_MAX_VOL_LEBS
 	int "Maximum number of LEBs per volume"
 	depends on SPL_UBI
@@ -677,6 +684,12 @@
 	help
 	  The UBI volume id from which to load U-Boot
 
+config SPL_UBI_LOAD_MONITOR_VOLNAME
+	string "volume name of U-Boot volume"
+	depends on SPL_UBI_LOAD_BY_VOLNAME
+	help
+	  The UBI volume name from which to load U-Boot
+
 config SPL_UBI_LOAD_KERNEL_ID
 	int "id of kernel volume"
 	depends on SPL_OS_BOOT && SPL_UBI