Add stable interface for setting host boot options

[adamlouis]

Callers, including production manufacturing software, currently rely on hiffy for setting host boot options. It would help to have a stable, committed interface here.

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Examples:

• https://github.com/oxidecomputer/facade/blob/54fa3ed7cf5d87e469de53db4c04885bd17ba607/scripts/cosmo_os.sh#L148

#
# The SP needs to be told, now, to boot from the network.
#
if ! "$HUMILITY" hiffy -c ControlPlaneAgent.set_startup_options \
    -a startup_options=0x80; then
	printf 'ERROR: setting system boot source failed.\n' >&2
	exit 1
fi

• https://github.com/oxidecomputer/dmar-report/blob/a2fdbc6b34ca5b19d25304d1697437a148469c85/dmar-runner.py#L115

    @retry(count=3, sleep=1)
    def enable_netboot(self):
        logging.info('Enabling net boot for target %s', self.target)
        cmd = ['pfexec', 'humility', '-t', self.target, 'hiffy',
               '-c', 'ControlPlaneAgent.set_startup_options', '-a',
               'startup_options=0x198']
        output_on_err(cmd)