dropbear: bump to version 0.53.1

Changelog @ http://matt.ucc.asn.au/dropbear/CHANGES



Since some build-time defaults have changed that make dropbear somewhat
bigger we introduce a new option to optimize dropbear for size.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>