package/dropbear: Fix symlink only if pointing to the right directory

This commit ensures that the /etc/dropbear symlink won't be removed if
it points elsewhere than /var/run/dropbear.

[Thomas:
 - fix indentation / too long lines as suggested by Yann E. Morin.]

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>