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>

include/ubispl.h

@@ -5,6 +5,8 @@
 #ifndef __UBOOT_UBISPL_H
 #define __UBOOT_UBISPL_H
 
+#define UBI_VOL_NAME_MAX	127
+
 /*
  * The following CONFIG options are relevant for UBISPL
  *
@@ -74,6 +76,10 @@ struct ubispl_info {
  */
 struct ubispl_load {
 	int		vol_id;
+#ifdef CONFIG_SPL_UBI_LOAD_BY_VOLNAME
+	u32		name_len;
+	char		name[UBI_VOL_NAME_MAX + 1];
+#endif
 	void		*load_addr;
 };