build: support multi config for build artefacts

Currently the rmm.elf, rmm.map, rmm.dump and rmm.img are created
in the top level `build` directory. This is a problem when using
multi config generators in Cmake. This patch modifies the artefact
path such that the build output is stored in a config specific
folder.

Signed-off-by: Soby Mathew <soby.mathew@arm.com>
Change-Id: Ieab3bfa4b88fd168571d98ef3b041c4820c554e5