fastboot: Increasing the max allowed download size

Creating a function which will return target specific max allowed flash size.

Change-Id: I7ff7d895af1cfea773e628c1a7a6b564043a3517
diff --git a/include/target.h b/include/target.h
index ace8834..c140d9d 100644
--- a/include/target.h
+++ b/include/target.h
@@ -32,6 +32,9 @@
 /* get memory address for fastboot image loading */
 void *target_get_scratch_address(void);
 
+/* get the max allowed flash size */
+unsigned target_get_max_flash_size(void);
+
 /* if target is using eMMC bootup */
 int target_is_emmc_boot(void);