init: Allow target-specific platform device base to be specified.

Add optional build-time flag, TARGET_PLATFORM_DEVICE_BASE,
to specify the base sysfs path (without the /sys prefix)
where the platform device resides.  This provides support for
targets where platform devices are all under a master level
platform device (e.g. soc-bus).
Change-Id: I1196fddaddbd03bd472db95ed4d67f818e3af0cb