uhttpd: use P-256 for certs

uhttpd: use P-256 for certs

The uhttpd package takes care of creating self-signed certificates if
px5g is installed. This improves the security of router management as it
encrypts the LuCI connection.

The EC P-256 curve is faster than RSA which which improves the user
experience on embedded devices. EC P-256 is support for as old devices
as Android 4.4.

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