installworld: Read-only objdirs not working

After a recent change a read-only /usr/obj, or other MAKEOBJDIRPREFIX,
will not properly work for an installworld.  I am working on a fix.

It will manifest as a missing rescue/rescue like:

> --- __installcheck_sh_check ---
> sh: /usr/src/rescue/rescue/rescue: not found
> rescue/sh check failed, installation aborted
> *** [__installcheck_sh_check] Error code 1

A workaround for now is to pass __objdir_writable=yes as a make argument:

    make installworld DESTDIR=... __objdir_writable=yes

This should only be used a temporary workaround and should be removed
once I commit a fix.

-- 
Regards,
Bryan Drewery