phosphor-gpio-monitor: Add EXTRA_ARGS in service

This is a follow up commit of 1065559c, that is to use ${EXTRA_ARGS}
instead of "--continue=", so that this option becomes an optional one
without argument.

For services need extra arguments, e.g. Romulus' id-button service,
specify "--continue" in its config file.

Tested: Verify the generated gpio monitor service uses ${EXTRA_ARGS}
        environment. On Romulus, checkstop monitor starts without extra
        argument, and id_button monitor starts with --continue argument.

Change-Id: I09c5ad7f97f1a40caabfc9ae394e68798396dffe
Signed-off-by: Lei YU <mine260309@gmail.com>
1 file changed