JDK-8048585 - Redesign SPI implementation to properly handle transactions

[Jen Dority]

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

Hi All,

I need a review for these changes done by an internal team to redesign 
the Serial Peripheral Interface implementation. The new code fixes some 
problems with the transaction handling and removes a lot of raspberry pi 
specific code.

Thanks,
Jen