Update dropbear to 2020.81

 Config changes: remove dss, add ed25519 keys, harden ciphers set

Changes applied it this patch:
- use modern dropbear version
- new version has separate config header file for customizations
- parameters in config are now set with 0/1 value instead undefined/defined
- update sets of ciphers, hashes, sighature algorithms, etc. to the stronge ones
- remove ssh host dss key and add ed25512 key in config and scripts
- add "--disable-harden" to dropbear compile options for it to succeed

Testing:
--------
Builds and runs for kvm_x86_64 and qemu_armv8a virtual targets.

X86:
 ssh to another host works.
 ssh to loopback interface works.

ARM64:
 ssh to another host works.
 ssh to localhost connects, but does not recognize the user.

However, the ARM64 localhost ssh fails in the same way that the
previous version of Dropbear did, so this upgrade does not seem to
break anything, either.

Signed-off-by: Alexander Zubkov <[email protected]>
Signed-off-by: Alex Doyle <[email protected]>