Add support to sign bootable images with vboot_signer

Add vboot properties to the dictionary file, which will be packed into
the target_files zip. Add support in packaging and OTA scripts to
sign the generated bootable images (boot.img and recovery.img) when
vboot is enabled.

Change-Id: I08758ced03d173219415bca762bbdb66c464a9f5
(cherry picked from commit 5d5a3bd9e8d8b14b71d1b2105417a2958d13d3d2)