add recovery update code to system images

Currently, the "img" zip files generated by the build system lack the
script and data needed to rewrite the recovery partition, while the
"ota" zip files do (when installed).

In order to move towards block-based OTAs, we want the result of
flashing an image and the result of installing the corresponding OTA
package to be identical.

Generate the recovery-from-boot patch and install script as part of
the process of building the target-files.  This requires breaking the
code to generate that out of ota_from_target_files into its own tool
that we can run from the Makefile.  (ota_from_target_files can still
do this, so it continues to work with older target-files.)

Bug: 12893978
Change-Id: I80e62268840780b81216e548be89b47baf81b4ac