app: aboot: Add UBI image awareness to fastboot code

Current implementation of flasher in LK just writes the provided image as raw data.
This is not good enough when writing a UBI image since some UBI META data should
be preserved.

Change-Id: I2c85b02d167db9a1a22a6eb6fe8181f7db1c64b2
diff --git a/target/init.c b/target/init.c
index c866187..9c850e5 100644
--- a/target/init.c
+++ b/target/init.c
@@ -54,6 +54,11 @@
     return (120 * 1024 * 1024);
 }
 
+__WEAK int flash_ubi_img(void)
+{
+    return 0;
+}
+
 __WEAK int target_is_emmc_boot(void)
 {
 #if _EMMC_BOOT