dropbear: new feature: disable-weak-ciphers

Enhances dropbear with a new feature "disable-weak-ciphers", on by default.
This feature disables all CBC, SHA1, and diffie-hellman group1 ciphers in
the dropbear ssh server and client.

Disable this feature if you need to connect to the ssh server from older
clients.  Additional customization can be done with local_options.h as usual.

Tested: On dropbear_2019.78.

Upstream-Status: Inappropriate [configuration]

Signed-off-by: Joseph Reynolds <joseph.reynolds1@ibm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>