Makefile: Add CONFIG_BUILD_TARGET to automatically build an special image

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>