Logging module with verbosity configurable in environment

This new module provides convenience macros that allow debug
logging for TRACE/DEBUG builds. Verbosity can be configured
via the environment variable HYBRIS_LOGGING_LEVEL (set to
"debug", "info", "warn", "error" or "disabled"). The default
logging level is "warn" (prints out warnings and errors).

Signed-off-by: Thomas Perl <m@thp.io>
hooks.c: Use new logging module for LOGD()

Signed-off-by: Thomas Perl <m@thp.io>
hybris/egl/platforms: Use logging module for TRACE()

Signed-off-by: Thomas Perl <m@thp.io>
Merge pull request #51 from thp/logging

Logging module