Add adb's ANDROID_SERIAL to fastboot too.

We noticed this when trying to script on-device testing for the dalvik
continuous build.

Also fix a typo in adb's help.
diff --git a/fastboot/fastboot.c b/fastboot/fastboot.c
index 8e92017..911c3f6 100644
--- a/fastboot/fastboot.c
+++ b/fastboot/fastboot.c
@@ -557,6 +557,8 @@
         return 0;
     }
 
+    serial = getenv("ANDROID_SERIAL");
+
     while (argc > 0) {
         if(!strcmp(*argv, "-w")) {
             wants_wipe = 1;