board: tbs2910: Autoselect environment device when booting from SD

Implement board specific functions to select the environment device and
partition when booting from SD/MMC.

SD2:  mmc 0 0
SD3:  mmc 1 0
eMMC: mmc 2 1

Signed-off-by: Soeren Moch <smoch@web.de>
diff --git a/include/configs/tbs2910.h b/include/configs/tbs2910.h
index 17b0213..0f23034 100644
--- a/include/configs/tbs2910.h
+++ b/include/configs/tbs2910.h
@@ -176,8 +176,8 @@
 
 /* Environment organization */
 #define CONFIG_ENV_IS_IN_MMC
-#define CONFIG_SYS_MMC_ENV_DEV		2
-#define CONFIG_SYS_MMC_ENV_PART		1
+#define CONFIG_SYS_MMC_ENV_DEV		2 /* overwritten on SD boot */
+#define CONFIG_SYS_MMC_ENV_PART		1 /* overwritten on SD boot */
 #define CONFIG_ENV_SIZE			(8 * 1024)
 #define CONFIG_ENV_OFFSET		(384 * 1024)
 #define CONFIG_ENV_OVERWRITE