Combine Build Scripts

Combine the aemfvp-a-rme/build-test-buildroot.sh and
aemfvp-a/build-test-busybox.sh into a single build script
aemfvp-a/build-stack.sh. This will alow simplification
of the build process and the associated documentation.

Signed-off-by: Ben Taylor <ben.taylor@linaro.org>
Change-Id: Ic1a183514611d2ed19dbb7f21d192a599960c3f1