dropbear/dropbearkey.service: Allow extra arguments for key generation

dropbear/dropbearkey.service: Allow extra arguments for key generation

Just like the "init" version this script, pass $DROPBEAR_RSAKEY_ARGS to the
'dropbearkey' program when generating a host key.

This allows to speed up SSH connections on read-only root systems by adding
the line DROPBEAR_RSAKEY_ARGS="-s 1024" into /etc/default/dropbear.

See also: c0efbcb47ab3 ("dropbear/init: Allow extra arguments for key generation")

Signed-off-by: Mike Looijmans <mike.looijmans@topic.nl>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>