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>
diff --git a/common/recipes-phosphor/gpio/phosphor-gpio-monitor/phosphor-gpio-monitor@.service b/common/recipes-phosphor/gpio/phosphor-gpio-monitor/phosphor-gpio-monitor@.service
index 63f88a1..8d0f4cd 100644
--- a/common/recipes-phosphor/gpio/phosphor-gpio-monitor/phosphor-gpio-monitor@.service
+++ b/common/recipes-phosphor/gpio/phosphor-gpio-monitor/phosphor-gpio-monitor@.service
@@ -4,5 +4,5 @@
 [Service]
 Restart=no
 EnvironmentFile={envfiledir}/obmc/gpio/%I
-ExecStart=/usr/bin/env phosphor-gpio-monitor --path=${{DEVPATH}} --key=${{KEY}} --polarity=${{POLARITY}} --target=${{TARGET}} --continue=${{CONTINUE}}
+ExecStart=/usr/bin/env phosphor-gpio-monitor --path=${{DEVPATH}} --key=${{KEY}} --polarity=${{POLARITY}} --target=${{TARGET}} ${{EXTRA_ARGS}}
 SyslogIdentifier=phosphor-gpio-monitor