Extend write-built-using to also generate the ${extra:Built-Using} substvar.

This makes it possible to track packages mentioned in the EXTRA_PACKAGES
variable (currently set to "bf-utf-source syslinux"). Thanks to Ansgar
Burchardt for the reports (Closes: #696418, #700026).

Note: Unknown packages are ignored, so architecture-specific packages
shouldn't be a problem.

Acked-by: Ian Campbell <ijc@debian.org>
Signed-off-by: Cyril Brulebois <kibi@debian.org>