config: Only warn if duplicate option/device comes from the same file

config: Only warn if duplicate option/device comes from the same file

This is useful for arm (possibly other arches too) where we want to have
a GENERIC kernel that only include files for the different SoC. Since
multiple SoCs/Board needs the same device we would need to do either :

    Include the device in a generic file
    Include the device in each file that really needs it

Option 1 works but if someone wants to create a specific kernel config
(which isn't uncommon for embedded system), he will need to add a lots
of nodevice to it.

Option 2 also works but produce a lots of warnings.

Reviewed by:	kevans
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D19424