dropbear: use https URLs

While a hash check is being done, it's still better to use a download
URL with HTTPS.

Signed-off-by: Danilo Bargen <mail@dbrgn.ch>
Reviewed-by: Adrian Perez de Castro <aperez@igalia.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>