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/meta-romulus/recipes-phosphor/gpio/id-button/obmc/gpio/id_button b/meta-romulus/recipes-phosphor/gpio/id-button/obmc/gpio/id_button
index ee966a5..b196357 100644
--- a/meta-romulus/recipes-phosphor/gpio/id-button/obmc/gpio/id_button
+++ b/meta-romulus/recipes-phosphor/gpio/id-button/obmc/gpio/id_button
@@ -2,4 +2,4 @@
 KEY=135
 POLARITY=1
 TARGET=id-button-pressed.service
-CONTINUE=true
+EXTRA_ARGS=--continue