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/target/init.c b/target/init.c
index 0fa8b8d..a771544 100644
--- a/target/init.c
+++ b/target/init.c
@@ -45,6 +45,11 @@
     return (void *)(SCRATCH_ADDR);
 }
 
+__WEAK unsigned target_get_max_flash_size(void)
+{
+    return (120 * 1024 * 1024);
+}
+
 __WEAK int target_is_emmc_boot(void)
 {
 #if _EMMC_BOOT