mbedtls: enable NIST curves optimisation.

luci using ustream-mbedtls is extremely slow vs ustream-polarssl.
polarssl alias mbedtls v1 is configured to use NIST prime speed
optimisation, so no longer disable the default optimisation for
mbedtls v2.

Compile & run tested: Archer C7v2

Signed-off-by: Kevin Darbyshire-Bryant <kevin@darbyshire-bryant.me.uk>
[Jo-Philipp Wich: refresh patch to use common format]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>