configs/{at91, atmel}*_dev*: drop Dropbear as it duplicates OpenSSH

configs/{at91, atmel}*_dev*: drop Dropbear as it duplicates OpenSSH

The "development" defconfigs for Atmel platforms enable both OpenSSH
and Dropbear, which doesn't make a lot of sense, as only one SSH
server can start on port 22.

This commit therefore drops BR2_PACKAGE_DROPBEAR=y from those
defconfigs, keeping OpenSSH as an SSH server/client, as was requested
by Atmel/Microchip folks in the review of an earlier version of this
patch [1]. Since those defconfigs are "development" defconfigs, they
are not meant to be minimal, and already provide an arbitrary set of
packages, so using openssh is just as good as using dropbear in this
case.

[1] https://patchwork.ozlabs.org/patch/989516/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Nicolas Ferre <nicolas.ferre@microchip.com>
Cc: Joshua Henderson <joshua.henderson@microchip.com>
Cc: Ludovic Desroches <ludovic.desroches@microchip.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>