Upgrading a removable disk installation

[Steve Bertrand]

Hi all,

I have a couple of systems that run from USB thumb sticks. These 
machines run 6.2.

I'd like to do a typical source upgrade to 7.0 on a test machine.

To do this, I plug the USB disk into a 'host' machine, and to save space 
on the thumb drive, I csup the sources to the standard locations on the 
host machine.

My thumb drive 'OS' slice is mounted as /var/removable-os on the host 
machine.

What I am unclear about is how I go about telling buildworld etc on the 
host machine. that I want to install the updates to the 
/var/removable-os location, instead of the default '/'.

I know how to do this with sysinstall, but I'm not sure if I need to put 
an entry into /etc/make.conf, or use a command-line switch to get my 
desired results.

Can someone please help clarify how I go about this?

Steve