Add comments to build files/documentation on the difference between agw_of vs mme_oai

[LKreutzer]

Add comments to build files/documentation on the difference between agw_of vs mme_oai. It may be confusing why there are two different targets for MME. Explain the difference and use-cases. See Makefile for ideas for a description - can we make it more precise than that?

# FEATURES: What kind of flavours do you want your MME or AGW have in it
# MME is MME as described in 3GPP specs, it has at least S1AP, S11, S6a
# interfaces.
# AGW means Acces GateWay, is the result of the aggregation of MME, SGW and PGW.
# First in FEATURES, select what to you want to build : mme or agw with OpenFlow
# (OVS): FEATURE=mme_oai or agw_of
# Then you can have other features that can be built for mme or agw :
# s6a with fd (freeDiameter)

# Default is agw with OpenFlow, gRPC over S6a , (no freeDiameter over s6a).
FEATURES ?= agw_of
# AVAILABLE_FEATURE_LIST : every feature not in this list will trigger an error.
AVAILABLE_FEATURE_LIST = agw_of mme_oai
REQUESTED_FEATURE_LIST = $(sort $(FEATURES))