Print config info about AVR_TOOLS_DIR always

In 25a31afbd92f03a1df1b50e3620f40afedd2e67e a bug was introduced which
prevented the config info for AVR_TOOLS_DIR to be printed when
AVRDUDE_CONF was defined.

This commit moves the config printing statement for AVR_TOOLS_DIR
outside the condition which checks whether AVRDUDE_CONF is defined.
Add CURRENT_OS macro which has the current os

Right now CURRENT_OS can have one of the following values

- WINDOWS
- LINUX
- MAC
Improve avrdude and avrdude_conf path auto detection

In Arduino Linux distribution, avrdude and avrdude.conf files are placed
in different path. This commit improves the autodetection of avrdude
and avrdude.conf paths by considering this fact.

Fix #48