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>