add/fix mcp23017 i2c gpio expander functions and tests

Change-Id: Ia2e5a1bf3f97272931014e54e587109297556c03
diff --git a/firmware/libboard/octsimtest/source/i2c.c b/firmware/libboard/octsimtest/source/i2c.c
index a708704..2cd8423 100644
--- a/firmware/libboard/octsimtest/source/i2c.c
+++ b/firmware/libboard/octsimtest/source/i2c.c
@@ -96,7 +96,7 @@
 	i2c_started = true;
 }
 
-static void i2c_stop_cond(void)
+void i2c_stop_cond(void)
 {
 	clear_sda();
 	set_scl();