let the "firmware" command take the file straight from the package

To do a firmware-install-on-reboot, the update binary tells recovery
what file to install before rebooting.  Let this file be specified as
"PACKAGE:<foo>" to indicate taking the file out of the OTA package,
avoiding an extra copy to /tmp.  Bump the API version number to
reflect this change.
diff --git a/updater/updater.c b/updater/updater.c
index 5a2ed2c..31d93ae 100644
--- a/updater/updater.c
+++ b/updater/updater.c
@@ -34,9 +34,10 @@
     }
 
     char* version = argv[1];
-    if (version[0] != '1' || version[1] != '\0') {
-        fprintf(stderr, "wrong updater binary API; expected 1, got %s\n",
-                version);
+    if ((version[0] != '1' && version[0] != '2') || version[1] != '\0') {
+        // We support version "1" or "2".
+        fprintf(stderr, "wrong updater binary API; expected 1 or 2, got %s\n",
+                argv[1]);
         return 2;
     }