arm: mvebu: mv-common.h: Add CONFIG_PREBOOT

This enables the usage of the "preboot" environment variable on Marvell
boards.

Signed-off-by: Stefan Roese <sr@denx.de>
Tested-by: Kevin Smith <kevin.smith@elecsyscorp.com>
Tested-by: Dirk Eibach <dirk.eibach@gdsys.cc>

include/configs/mv-common.h

@@ -57,6 +57,7 @@
 					  115200,230400, 460800, 921600 }
 /* auto boot */
 #define CONFIG_BOOTDELAY	3	/* default enable autoboot */
+#define CONFIG_PREBOOT
 
 /*
  * For booting Linux, the board info and command line data