Add concept of console output log levels

Create new LOG_LEVEL build option, which controls the amount of
console output compiled into the build. This should be one of the
following:

    0  (LOG_LEVEL_NONE)
    10 (LOG_LEVEL_NOTICE)
    20 (LOG_LEVEL_ERROR)
    30 (LOG_LEVEL_WARNING)
    40 (LOG_LEVEL_INFO)
    50 (LOG_LEVEL_VERBOSE)

All log output up to and including the log level is compiled into the
build. The default value is 40 in debug builds and 20 in release
builds.

Complement the existing INFO, WARN and ERROR console output macros
with NOTICE and VERBOSE macros, which are conditionally compiled in
depending on the value of LOG_LEVEL.

Fixes ARM-software/tf-issues#232

Change-Id: I951e2f333e7b90fc4b1060741d9a6db699d5aa72