dropbear: fix dropbear startup issue

dropbear: fix dropbear startup issue

Interface triggers are installed by the dropbear init script in case an
interface is configured for a given dropbear uci section.
As dropbear is started after network the interface trigger event can be
missed during a small window; this is especially the case if lan is
specified as interface.
Fix this by starting dropbear before network so no interface trigger
is missed. As dropbear is started earlier than netifd add a boot function
to avoid the usage of network.sh functions as call to such functions will
fail at boottime.

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>
Acked-by: Jo-Philipp Wich <jo@mein.io>