[VOL-3548] Differentiate between OLT gRPC channel disconnect and device reboot

- Return the previously_connected state flag in response to GetDeviceInfo to VOLTHA

Change-Id: I3a39eccdda386658226c6379a4815900b808795d
diff --git a/agent/Makefile.in b/agent/Makefile.in
index 5ccea59..50636d7 100644
--- a/agent/Makefile.in
+++ b/agent/Makefile.in
@@ -42,7 +42,7 @@
 # This specifies the GIT tag in https://github.com/opencord/voltha-protos
 # repo that we need to refer to, to pick the right version of
 # openolt.proto and tech_profile.proto
-OPENOLT_PROTO_VER ?= v4.0.14
+OPENOLT_PROTO_VER ?= v4.0.15
 
 # Variables used for Inband build
 INBAND = "n"