core: Initial implementation for run-time selection of log level

Log level can be controlled using NFD_LOG environment variable. The
following options are available

Set up log level using mnemonic

    NFD_LOG=none|error|warn|info|debug|trace

Set up log level using log-level number (0 none, >=5 all)

    NFD_LOG=0|1|2|3|4|5|6|...

Change-Id: Id38c2775f1137a4e38e4896963ab9a01fc863d2c
Refs: #1267