Provide a working GPIOBUS_IVAR() macro for FDT systems.
Hook libxo to the build.

Sponsored by:	Juniper Networks, Inc.
Hook xo(1) to the build -- it's like echo, but uses libxo to
support emitting machine-readable output.

Sponsored by:	Juniper Networks, Inc.
Move the duplicated code to a single function.

No functional changes.