build-all: allow component build scripts to override default build configs

The existing method of overriding default build configurations by calling
into __do_override_build_configs is useable only within the build-all.sh
script. For component build scripts, __do_override_build_configs is not
accessible and so component build script cannot override the default build
configuration. So export the declaration of the override function if it is
available so that component build scripts can override the default build
configurations.

Change-Id: I62621a2f59117541f71fe11b77a1b005fc030f62
Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>