Fix test-dts{,o} targets

There were two main problems here:

1.) sys/dts/Makefile.inc is not included from various */overlays directories

by default, only ../Makefile.inc

2.) When shelling out for DTS/DTSO, cwd != .CURDIR, so enumeration always

failed

These changes allow make test-dts and make test-dtso to function in their
respective directories.

Reviewed by: manu
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D17961