target: include: Fix for UFS support

Add weak function to return boot storage device & change prototype
for target_mmc_device function

Change-Id: I46abb039602bb5181b4af907394a18e985f8bd64
diff --git a/include/target.h b/include/target.h
index 45f1605..00f5885 100644
--- a/include/target.h
+++ b/include/target.h
@@ -48,7 +48,7 @@
 void target_fastboot_init(void);
 void target_load_ssd_keystore(void);
 bool target_is_ssd_enabled(void);
-struct mmc_device *target_mmc_device();
+void *target_mmc_device();
 
 bool target_display_panel_node(char *pbuf, uint16_t buf_size);