dropbear: read configuration from /etc/default/dropbear

Patch allows to specify additional parameters (e.g. '-B') by reading
/etc/default/dropbear as a systemd environment file.  It applies
$DROPBEAR_EXTRA_ARGS which used already by the oe-core sysv initscript.

Signed-off-by: Enrico Scholz <enrico.scholz@sigma-chemnitz.de>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>