Move the MAKEOBJDIRPREFIX value guard to sys.mk and expand to MAKEOBJDIR.

Move the MAKEOBJDIRPREFIX value guard to sys.mk and expand to MAKEOBJDIR.

This will ensure that the variable was not set as a make override, in
make.conf, src.conf or src-env.conf.  It allows setting the value in
src-env.conf when using WITH_AUTO_OBJ since that case properly handles
changing .OBJDIR (except if MAKEOBJDIRPREFIX does not yet exist which is
being discussed to be changed).

This change allows setting a default MAKEOBJDIRPREFIX via local.sys.env.mk.

Sponsored by:	EMC / Isilon Storage Division