dropbear: make rsa-sha2-256 pubkeys usable again

dropbear: make rsa-sha2-256 pubkeys usable again

Upstream in commit 972d723484d8 ("split signkey_type and signature_type
for RSA sha1 vs sha256") has added strict checking of pubkey algorithms
which made keys with SHA-256 hashing algorithm unusable as they still
reuse the `ssh-rsa` public key format. So fix this by disabling the
check for `rsa-sha2-256` pubkeys.

Ref: https://tools.ietf.org/html/rfc8332#section-3
Fixes: d4c80f5b172e ("dropbear: bump to 2020.80")
Tested-by: Russell Senior <russell@personaltelco.net>
Signed-off-by: Petr Štetiar <ynezz@true.cz>