core/Makefile: Allow specification of file locations

If multiple build targets use a common recovery.fstab or system.prop,
this allows them to live in a centralized location and not be copied
or symlinked to the device directories.

Change-Id: I7aeb31ed9dc9b43a5e2c9e46fe582b10cba71b24
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>