openssh-keygen: Make ssh-keygen as an alternative to dropbearkey

openssh-keygen: Make ssh-keygen as an alternative to dropbearkey

The DropBear's dropbearkey tool is compatible with OpenSSH
ssh-keygen.
It was set by default as the /usr/bin/ssh-keygen program since
the PR https://github.com/openwrt/openwrt/pull/14174
Now if a user need for a full ssh-keygen the openssh-keygen package
should substitute it gracefully as an alternative.

Signed-off-by: Sergey Ponomarev <stokito@gmail.com>
Link: https://github.com/openwrt/packages/pull/22861
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Link: https://github.com/openwrt/packages/pull/22861
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>