build_image: Use new build flags for factory installer and recovery images.

Apply the new use flag 'factory_shim_ramfs' and 'recovery_ramfs' to select the
right initramfs file.

BUG=chromium:425938,chrome-os-partner:36851
TEST=build_image factory_install # success.
     mod_image_for_recovery.sh # success.
CQ-DEPEND=CL:248434,CL:253181

Change-Id: I80bbeeb1053f1028a401e7aa98b5b2945c5ae250
Reviewed-on: https://chromium-review.googlesource.com/248551
Tested-by: Hung-Te Lin <hungte@chromium.org>
Reviewed-by: Gaurav Shah <gauravsh@chromium.org>
Commit-Queue: Hung-Te Lin <hungte@chromium.org>
2 files changed