Platform/RaspberryPi: move varstore structure defs to ConfigVars.h

To avoid hardcoding constants for non-obvious fields (e.g. enum
instead of basic enable/disable),  move variable structure and value
definitions into a separate header, ConfigVars.h.

Signed-off-by: Andrei Warkentin <andrey.warkentin@gmail.com>
Reviewed-by: Pete Batard <pete@akeo.ie>