Rename vbmeta_mainline to vbmeta_system.

Bug: 116859651
Test: fastboot flash vbmeta_system
Change-Id: Ice65b6f5141ec6da3e1c91d6a3aac58533dc2314
diff --git a/fastboot/fastboot.cpp b/fastboot/fastboot.cpp
index 5962650..e27a897 100644
--- a/fastboot/fastboot.cpp
+++ b/fastboot/fastboot.cpp
@@ -155,10 +155,10 @@
     { nullptr,    "system_other.img", "system.sig",   "system",   true,  ImageType::Normal },
     { "userdata", "userdata.img",     "userdata.sig", "userdata", true,  ImageType::Extra },
     { "vbmeta",   "vbmeta.img",       "vbmeta.sig",   "vbmeta",   true,  ImageType::BootCritical },
-    { "vbmeta_mainline",
-                  "vbmeta_mainline.img",
-                                      "vbmeta_mainline.sig",
-                                                      "vbmeta_mainline",
+    { "vbmeta_system",
+                  "vbmeta_system.img",
+                                      "vbmeta_system.sig",
+                                                      "vbmeta_system",
                                                                   true,  ImageType::BootCritical },
     { "vendor",   "vendor.img",       "vendor.sig",   "vendor",   true,  ImageType::Normal },
     { nullptr,    "vendor_other.img", "vendor.sig",   "vendor",   true,  ImageType::Normal },