MFC r249906:

Use the system MAKEOBJDIRPREFIX when running make targets in
mergemaster.  This allows bootstrap verions of tools to be used.

Add a note to UPDATING about this change.

This commit is a partial backout of r248531 in that it removes an
accidentally committed change to mergemaster.

Discussed with:	jhb
Sponsored by:	DARPA, AFRL