Use ALL-y style instead of ifeq blocks for better readability

Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@googlemail.com>
diff --git a/board/spear/spear600/config.mk b/board/spear/spear600/config.mk
index 11da2c3..5848ef8 100644
--- a/board/spear/spear600/config.mk
+++ b/board/spear/spear600/config.mk
@@ -25,7 +25,7 @@
 
 CONFIG_SYS_TEXT_BASE = 0x00700000
 
-ALL += $(obj)u-boot.img
+ALL-y += $(obj)u-boot.img
 
 # Environment variables in NAND
 ifeq ($(ENV),NAND)