Don't kill sdcards with boot_tries

USB/SDCard boot devices typicaly don't run software updater,
and won't have success ever set. We never want to try to
fall back to the empty B partition either. The workaround
for this is to set the successful bit regardless of actual
boot success.

bug=6395
Change-Id: I67c625804203b13be9a0c626c404fa38bafb5445

Review URL: http://codereview.chromium.org/3344008

build_image

@@ -651,6 +651,10 @@ create_base_image() {
     "${OUTPUT_DIR}" \
     "${OUTPUT_DIR}/${image_name}"
 
+  # Pre-set "sucessful" bit in gpt, so we will never mark-for-death
+  # a partition on an SDCard/USB stick.
+  ${GPT} add -i 2 -S 1 "${OUTPUT_DIR}/${image_name}"
+
   trap - EXIT
 }