Follow-up r318105: Don't use NO_OBJ at top-level, use NO_OBJWALK.
NO_OBJ has a very specific meaning in sub-directories in that no object
directory will be made. If a user wanted to skip the 'make obj' phase then
passing -DNO_OBJ would break all sub-directories from building properly. Using
NO_OBJ internally also causes issue with NO_OBJ handling being added in
share/mk/bsd.init.mk soon.
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