d/prerm: add prerm script

Lets add the prerm script for fw-admin.

Signed-off-by: Arturo Borrero Gonzalez <aborrero@cica.es>
d/postrm: add postrm script

Add the postrm script for fw-admin.

Signed-off-by: Arturo Borrero Gonzalez <aborrero@cica.es>