CMakeLists.txt: add DEBUG define when doing a debug build

Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>
mraa_internal.h: fix styling issues

Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>
syslog: remove all fprintf calls and use syslog instead

Syslog is now used for all error messages, return values in the code should be
used by programmers to see the status of the library/board and syslog can be
used to see quickly from a debugging perspective what has gone wrong. A few
cosmetics where improved as well as a mraa_set_log_level() call where the
syslog log mask can be set directly from libmraa.

Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>
index.md: add ADC change for 0.5.0

Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>