API: Convert conditional building to the new scheme.

This fixes a build breakage with CONFIG_API enabled, which appeared after
the recent changes in the U-Boot build system.

Signed-off-by: Rafal Jaworowski <raj@semihalf.com>
diff --git a/api/Makefile b/api/Makefile
index 94de3dc..4216892 100644
--- a/api/Makefile
+++ b/api/Makefile
@@ -24,13 +24,12 @@
 
 LIB	= $(obj)libapi.a
 
-COBJS	= api.o api_net.o api_storage.o api_platform-$(ARCH).o
+COBJS-$(CONFIG_API) += api.o api_net.o api_storage.o api_platform-$(ARCH).o
 
+COBJS	:= $(COBJS-y)
 SRCS	:= $(COBJS:.o=.c)
 OBJS	:= $(addprefix $(obj),$(COBJS))
 
-all:	$(LIB)
-
 $(LIB):	$(obj).depend $(OBJS)
 	$(AR) $(ARFLAGS) $@ $(OBJS)