POSIX: Improved logging

The warnx and warn calls map to PX4_WARN.
Calls to errx or err genrtate a compile error.

The px4_log.h file implements a new log format:

For DEBUG and INFO:
<level> <msg>

For ERROR and WARN:
<level> <msg> (file filepath line linenum)

The verbosity can be changed by setting the macro to use
either linux_log or linux_log_verbose in px4_log.h

Signed-off-by: Mark Charlebois <charlebm@gmail.com>