Set the runtime platform for Pi4J to use. This platform selection
will be used to determine the default GPIO providers and I2C providers
specific to the selected platform. A platform assignment can only
be set once. If a second attempt to set a platform is attempted,
the 'PlatformAlreadyAssignedException' will be thrown. Please
note that platform assignment can be made automatically if you
use a provider class prior to manually assigning a platform.
Internal method to get the default platform. It will attempt to first
get the platform using the 'PI4J_PLATFORM' environment variable, if the
environment variable is not configured, then it will attempt to use the
system property "pi4j.platform". If the system property is not found or
the value is not legal, then return the default 'RASPBERRY_PI' platform.