Flash cache partition with cache.img by default.

Bug: 5153694
Change-Id: If322d13286e9b0f14877d1a4f03f6bd288ee71f3
diff --git a/fastboot/fastboot.c b/fastboot/fastboot.c
index 4a2de20..0639f3b 100644
--- a/fastboot/fastboot.c
+++ b/fastboot/fastboot.c
@@ -88,6 +88,8 @@
         fn = "system.img";
     } else if(!strcmp(item,"userdata")) {
         fn = "userdata.img";
+    } else if(!strcmp(item,"cache")) {
+        fn = "cache.img";
     } else if(!strcmp(item,"info")) {
         fn = "android-info.txt";
     } else {