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.
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