dropbear: fix missing directory with systemd

The current SysV startup script create a directory which is necessary
for dropbear to correctly work.
This creation is not done with systemd.

Instead of both init creating the directory, we add the creation of this
directory to the INSTALL_TARGET_CMDS to make sure it's present.

[Peter: use make syntax for TARGET_DIR as pointed out by Thomas]
Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>