Add option to set an external arduino libraries directory.
All libraries from this folder will be compile and available for use
in the gateway.
Some Arduino libraries can be used on Linux/RPI with minor adjustments,
but there are some that will have to be completely rewritten.
The build process uses a separate folder for objects and binary files
which can be set by the user.
Initialize the bcm2835 inside a class constructor to fix cases where
pin operations are used before hwInit().
Fix SPI pins default values.
Fix activation interrupt functions.
Fix some headers.
Rename gateway binary and services to mysgw.
