RFR: JDK-8053996 JDK-8051815 : Make spibus, i2c, and uart optional in dio build & Change DIO src layout to improve modularization of native code

[Jen Dority]

issue: JDK-8053996 <https://bugs.openjdk.java.net/browse/JDK-8053996> 
JDK-8051815 <https://bugs.openjdk.java.net/browse/JDK-8051815>
webrev: http://cr.openjdk.java.net/~jld/8053996a/

Hi All,

I am submitting two JIRA issues in one webrev because *815 (minor source 
code reorg) was needed for *996 (add ability to leave some APIs out of 
the build).

The webrev looks much scarier than it really is. All the green and red 
is just files being pushed under another layer of directories and one 
include line being changed (in serial.c) to reflect this move.

The rest of the changes are specifically for *996 and are in the 
top-level Makefile to be able to strip out specific APIs in the build.

Kindly review.

Thanks,
Jen