Let user skip MONITOR_PORT existance check

Define FORCE_MONITOR_PORT to enable.
Useful if one uses 'ssh root@remotemachine avrdude' instead of the
usual AVRDUDE command.
Let 'build.core' from boards.txt change ARDUINO_CORE_PATH

This is support for https://code.google.com/p/arduino-tiny
The arduino-tiny project provides a boards.txt file and a whole
separate Arduino core modified to work with attinies.
Arduino.mk will now switch to that core if it finds a
'build.core' parameter in boards.txt and a folder in
$(ALTERNATE_CORE_PATH)/cores by that name.
Example: set fuses and upload hex on remote programmer

Implements two new make rules: 'net_set_fuses' executes a single
ssh command, 'net_upload' pipes hex through ssh connection.

The example also showcases the FORCE_MONITOR_PORT feature.
Update changelog
Note on using root in BlinkNetworkRPi
Tidy new arduino mk vars

Add CORE and FORCE_MONITOR_PORT to arduino-mk-vars.md and FORCE_MONITOR_PORT
to config print.
Typo
Fix build.core parsing

Disregard anything before first colon.
E.g. build.core = arduino:arduino results in CORE = arduino
Print CORE.
Merge pull request #307 from Gaboose/pr

Ease flashing through ssh. Tweak alternate core support. #306 rebased