RFR: JDK-8048585 - Redesign SPI implementation to properly handle transactions

[Alexey Konstantinov]

issue: https://bugs.openjdk.java.net/browse/JDK-8048585
webrev: http://cr.openjdk.java.net/~alkonsta/8048585/webrev

Hello All,

Please take a look at the changes made to refactor the SPI 
implementation in order to handle transactions properly and get rid of 
the GPIO dependencies and the raspberry pi hardcoded stuff.

Thanks,
Alexey