phosphor-reboot-host: Use environment file for sleep

By default phosphor-reboot-host@.service sleeps for 5 seconds between
power off and power on.
Make the sleep time as configurable parameter in environment, so
different machines could use different sleep time.

Tested: Verify the sleep time is from the config file.

Change-Id: I81d17e56010fe595dd46295923746f8678678209
Signed-off-by: Lei YU <mine260309@gmail.com>
diff --git a/common/recipes-phosphor/state/files/obmc/phosphor-reboot-host/reboot.conf b/common/recipes-phosphor/state/files/obmc/phosphor-reboot-host/reboot.conf
new file mode 100644
index 0000000..2e3ca1a
--- /dev/null
+++ b/common/recipes-phosphor/state/files/obmc/phosphor-reboot-host/reboot.conf
@@ -0,0 +1 @@
+REBOOT_DELAY=5