test block system image patch at build time

After building a patch for the system image (for incremental block
OTAs), apply it to a local copy of the file and test that it succeeds.
This is an imperfect test as it's using the local client's
syspatch_host, which may differ from the syspatch library actually
used in the target build, but it's somewhat better than nothing.

Change-Id: Ic0001b0145881e2ebd4b5b36ce9b5bcebd76deb4