Defaults to the directory determined by the _SRC_TOP_ logic (if != ""),
and the BSDSRCDIR.
NETBSDSRCDIR has been provided for use by the various NetBSD source
Makefiles to find the top of the NetBSD source tree, and isn't
affected by the inheritance properties of _SRC_TOP_, nor does it
have the magic BSDOBJDIR baggage that BSDSRCDIR is stuck with.
Use the old setup for MAKEOBJDIRPREFIX but also add a new check for
_SRC_TOP_OBJ_ and use that if it's set. This allows a make release using
build.sh (which uses MAKEOBJDIR patterns) to function correctly on r/o
source tree's.
lukem