mbedtls: Activate secp521r1 curve by default

mbedtls: Activate secp521r1 curve by default

Activate the secp521r1 ecliptic curve by default. This curve is allowed
by the CA/Browser forum, see
https://cabforum.org/wp-content/uploads/CA-Browser-Forum-BR-v2.0.1-redlined.pdf#page=110

This increases the size of libmbedtls12_2.28.5-1_aarch64_generic.ipk by
about 400 bytes:
Without:
252,696 libmbedtls12_2.28.5-1_aarch64_generic.ipk
With:
253,088 libmbedtls12_2.28.5-2_aarch64_generic.ipk

Fixes: #13774
Acked-by: Koen Vandeputte <koen.vandeputte@citymesh.com>
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>