Add openSUSE perl package hints to README.md.

Signed-off-by: Ralf Doering <ralf@rdoering.net>
Don't overwrite AVRDUDE_CONF if AVR_TOOLS_DIR is not set.

If one likes to use avrdude from outside of Arduino together with
Arduino-bundled gcc crosstolchain, the setting of AVRDUDE_CONF got
overwritten.
Mention AVRDUDE and AVRDUDE_CONF in README.md.
Merge pull request #90 from ralfdoering/opensuse

Documention and fixes for openSUSE and system provided avrdude
Update instructions about including libraries
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
Move binary sketch size verification logic inside makefile

Earlier bin/ard-verify-size shell script was used to validate that the
binary hex file size is less than the flash memory of the target
microcontroller.

This commit removes the dependency on the external shell script, by
moving the binary size verification logic inside the makefile itself.

Fix #54