disk: Add an option for partitions in SPL

In some cases we do not want to enable partition support in SPL. Add an
option to allow this.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/disk/Makefile b/disk/Makefile
index 6ce5a68..b197692 100644
--- a/disk/Makefile
+++ b/disk/Makefile
@@ -5,7 +5,7 @@
 
 #ccflags-y += -DET_DEBUG -DDEBUG
 
-obj-$(CONFIG_PARTITIONS)	+= part.o
+obj-$(CONFIG_$(SPL_TPL_)PARTITIONS)  += part.o
 obj-$(CONFIG_$(SPL_)MAC_PARTITION)   += part_mac.o
 obj-$(CONFIG_$(SPL_)DOS_PARTITION)   += part_dos.o
 obj-$(CONFIG_$(SPL_)ISO_PARTITION)   += part_iso.o