build-hafnium.sh: copy if local build configs are present before build

Allow hafnium build script to have a provision to copy local build
config files instead of using from the upstream reference repo. This
allow hafnium build scripts to pick build config files that are not
upstreamed yet. The script checks for the config files the respective
platform's config path and copy them to the project/reference path of
hafnium before initiating the build.

Signed-off-by: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>
Change-Id: Ife769d05da7cb7dd211b58c62d0f884bc01fca36