dropbear: bump to 2020.79

dropbear: bump to 2020.79

- drop patches (applied upstream):
  * 010-backport-change-address-logging.patch
  * 020-backport-ed25519-support.patch
  * 021-backport-chacha20-poly1305-support.patch
- backport patches:
  * 010-backport-disable-toom-and-karatsuba.patch:
    reduce dropbear binary size (about ~8Kb).
- refresh patches.
- don't bother anymore with following config options
  because they are disabled in upstream too:
  * DROPBEAR_3DES
  * DROPBEAR_ENABLE_CBC_MODE
  * DROPBEAR_SHA1_96_HMAC
- explicitly disable DO_MOTD as it was before commit a1099ed:
  upstream has (accidentally) switched it to 0 in release 2019.77,
  but reverted back in release 2020.79.

Signed-off-by: Konstantin Demin <rockdrilla@gmail.com>