build: Move EXTRA_DIST away from build.rules.mk into build.dist.mk

This allows for optional inclusion of build.rules.mk without needing to redefine
EXTRA_DIST for the case where the include does not happen. Also fixes the issue
where theme icons for $component don't appear in the tarball when
--disable-$component happens.

Signed-off-by: Bertrand Lorentz <bertrand.lorentz@gmail.com>