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: I7b19b69f2c95335d6512e181f265b874e1713f64