gnu: Move flashrom and avrdude to (gnu packages flashing-tools).

* gnu/packages/admin.scm (flashrom): Move this and...
* gnu/packages/avrdude.scm (avrdude): ... this to...
* gnu/packages/flashing-tools.scm: ... this. New file.
* gnu-system.am (GNU_SYSTEM_MODULES): Adjust accordingly.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>