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