Export SB_OBJROOT for later reference

The build varies MAKEOBJDIRPREFIX at times which
can make it difficult to track the original OBJROOT.
This is easier when we get the original from env as SB_OBJROOT.
If SB_OBJROOT did not come from env, set and export it
the first time we set OBJROOT.

This allows SB_OBJROOT to be used in .MAKE.META.IGNORE* variables
to tweak what should be considered as making a target out-of-date.

Update auto.obj.mk to latest from contrib/bmake - handles relative
objdirs more simply and reliably.

Also update jobs.mk and stage-install.sh

Reviewed by: stevek
Differential Revision: https://reviews.freebsd.org/D50313