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Add target to build it automatically upon "make" / MAKEALL. This can/should
be set by board / cpu specific headers if a special U-Boot image is
required for this SoC / board.

E.g. used by Marvell Armada XP to automatically build the u-boot.kwb
target.
Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Masahiro Yamada <yamada.m@jp.panasonic.com>