distro_boot: add a new build script for distro boot test

For disto boots, the entire stack need not be built. The components
required to be built are scp, arm-tf and uefi. Add a new build
script to build these components for distro boot test. This script
is a wrapper on the build-all script.

Change-Id: I19a44eb37acc1d8923a47c11d78a20122f922e81
Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>