dropbear: Enable Ed25519 for normal devices

dropbear: Enable Ed25519 for normal devices

The Ed25519 key pairs are much shorter than RSA pairs and are supported
by default in OpenSSH. Looking at websites explaining how to create new
SSH keys, many suggest using Ed25519 rather than RSA, however consider
the former as not yet widely established. OpenWrt likely has a positive
influence on that development.

As enabling Ed25519 is a compile time option, it is currently not
possible to install the feature via `opkg` nor select that option in an
ImageBuilder.

Due to the size impact of **12kB** the option should only be enabled for
devices with `!SMALL_FLASH`.

This approach seems cleaner than splitting `dropbear` into two packages
like `dropbear` and `dropbear-ed25519`.

Signed-off-by: Paul Spooren <mail@aparcar.org>