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
Rationalize console log output

Fix the following issues with the console log output:

* Make sure the welcome string is the first thing in the log output
(during normal boot).
* Prefix each message with the BL image name so it's clear which
BL the output is coming from.
* Ensure all output is wrapped in one of the log output macros so it can
be easily compiled out if necessary. Change some of the INFO() messages
to VERBOSE(), especially in the TSP.
* Create some extra NOTICE() and INFO() messages during cold boot.
* Remove all usage of \r in log output.

Fixes ARM-software/tf-issues#231

Change-Id: Ib24f7acb36ce64bbba549f204b9cde2dbb46c8a3