Changes

config/sys.config: print prefix before msg, if present

It's sometimes desirable to add some context to logging messages,
like the FSM name and identifier, for instance.  With this patch,
this can be easily achieved by passing an arbitrary prefix string
to a logging function/macro:

  %% EXAMPLE: adding metadata to a specific log macro
  ?LOG_INFO("The planet is fine...", #{prefix = "EARTH"}).

or by setting it for all logging of a process:

  %% EXAMPLE: adding context for the whole process
  logger:set_process_metadata(#{prefix => "E-RAB FSM"}).

Change-Id: Ib88801b8d38976b98a02ed06e1638e0e9056bafb

Makefile: allow overriding the config file

Change-Id: If077e9c0e06d7b081b5254bcded77ee65bcd8e5d

rebar.{config,lock}: add pfcplib to deps[]

Change-Id: I4cf3b3b09b19ebabbaad082c616c31ffc1327fd7