mainapp: Parse action -> target mappings on the command line

This adds the argument --action_target=<action>=<systemd_target>.
Depending on the HostAction that is set when the watchdog expires we can
start different systemd targets. The configuration of this mapping is
now configurable at the command line.

Change-Id: I7482e038d9abafc81cec0df15345cbfc670dbed2
Signed-off-by: William A. Kennington III <wak@google.com>
2 files changed