Fix an argument error in the keyword

It should use the argument '${i_model}' rather than the global variable
${OPENBMC_MODEL} in the keyword.

Signed-off-by: Tony Lee <tony.lee@quantatw.com>
Change-Id: Ic681d96f9d34045f3e7c5f90eb6d48aa63ee05ab
diff --git a/lib/serial_connection/serial_console_client.robot b/lib/serial_connection/serial_console_client.robot
index 293ff4f..da3fb39 100644
--- a/lib/serial_connection/serial_console_client.robot
+++ b/lib/serial_connection/serial_console_client.robot
@@ -22,7 +22,7 @@
     # i_port    The port of the serial console.
     # i_model   The path to the system data, i.e. "./data/Witherspoon.py".
 
-    ${prompt_string}=  Convert To Lowercase  ${OPENBMC_MODEL} login:
+    ${prompt_string}=  Convert To Lowercase  ${i_model} login:
     Telnet.Open Connection
     ...  ${i_host}  port=${i_port}  prompt=#
     Telnet.Set Timeout  30 seconds