Replacing openssh buildout by dropbear one.

Adding my own dropbear patch which was submitted to
dropbear developpers.

Add ipv6 patch for dropbear

Took here :
<https://lists.openwrt.org/pipermail/openwrt-devel/2009-May/004299.html>

Minor: Dropbear buildout fine tuning

Linking dropbear with slapos zlib