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>
diff --git a/Makefile b/Makefile
index 25161e6..bd0f45e 100644
--- a/Makefile
+++ b/Makefile
@@ -758,6 +758,11 @@
 endif
 endif
 
+# Add optional build target if defined in board/cpu/soc headers
+ifneq ($(CONFIG_BUILD_TARGET),)
+ALL-y += $(CONFIG_BUILD_TARGET:"%"=%)
+endif
+
 LDFLAGS_u-boot += $(LDFLAGS_FINAL)
 ifneq ($(CONFIG_SYS_TEXT_BASE),)
 LDFLAGS_u-boot += -Ttext $(CONFIG_SYS_TEXT_BASE)