framework: allow overriding of default build configuration

Allow overriding of the default build configuration before calling
the clean/build/package stages of the component build scripts. This
is acheived by calling the __do_override_build_configs function
which can be optionally implemented by the test case build script.

Change-Id: I4f833df7582751aa50a1a945fc51a41d05f033b5
Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>