On top of making <component>_CUSTOM much more complicated to rewrite,
you couldn't depend on a requirement for all of the components to treat
custom sources the same with it around.
If you need a custom source for a component, define it in the component.
Signed-off-by: Bryan Hundven <bryanhundven@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>