dropbear: Fix host key loading with 521 bit ecdsa keys

dropbear: Fix host key loading with 521 bit ecdsa keys

Dropbear 2018.76 changed the default ecdsa host key size form 521 to 256
bits, but this breaks systems with an existing 521 bit key, blocking ssh
logins.

Apply the upstream fix from https://secure.ucc.asn.au/hg/dropbear/rev/0dc3103a5900 :

  Only advertise a single server ecdsa key when -R (generate as required) is
  specified. Fixes -R now that default ecdsa key size has changed.

[Peter: apply-patches.sh does not like suffix-less filename, so include
	patch in Buildroot]
Signed-off-by: Stefan Sørensen <stefan.sorensen@spectralink.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>