scons: Get rid of the PHONY_BASE construction variable.

scons: Get rid of the PHONY_BASE construction variable.

The value of that variable can be computed more directly and more locally
to where it's consumed.

Change-Id: I5ca1f732a34e22d4dae2aeb6ee7fc8adebe1caa0
Reviewed-on: https://gem5-review.googlesource.com/2981
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Curtis Dunham <curtis.dunham@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>