dropbear: bump to version 2013.56

Switch ECHOs to SEDs to avoid issues with defines changing after
sysoptions.h was included - and it's cleaner.
Enable hmac-sha2-256 and hmac-sha2-512 support for the full featured.

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