Enable SHA2_256_HMAC and SHA2_512_HMAC in dropbear

Dropbear in cero does not support SHA2 HMACs.

root@cerowrt:/etc# ssh -m help
ssh: Available MACs:
hmac-sha1-96,hmac-sha1,hmac-md5

The objective of the patch is to enable SHA2 HMACs that exist since 2013.56.
The purpose of the patch is to allow ssh communication with hosts that do not support SHA1 or MD5 MACs as suggested by bettercrypto.org folks.

Please review.
Maciej