Currently three different tool chains seem to be used:
* avr-*
* pic32-*
* arm-none-eabi-*
These all get set up independently. This patch centralizes the
definitions of the executable tools and does it generically, by means
of the newly introduced TOOL_PREFIX variable. Setting up a
tool chain is now simply a matter of defining the TOOL_PREFIX
variable. For the currently supported tool chains it gets set to avr,
pic32 or arm-none-eabi. Arbitrary tool chains can now easily be set up,
by the TOOL_PREFIX variable.
Although the use of the OVERRIDE_EXECUTABLES variable is now almost
not justifiable, it was left as-is, in order to assure backwards
compatibility.
There seems to be 3 different macros to parse the boards.txt file.
This patch moves the PARSE_BOARD macro from Arduino.mk to Common.mk.
The PARSE_OPENCM and PARSE_TEENSY macros in Teensy.mk and OpenCM.mk
were removed and the common PARSE_BOARD is now being called from
everywhere.
Advantages of this fix are:
1. Less code, i.e. no redundant parse macros.
2. A single standardized algorithm to parse the boards.txt file.
