px5g: support EC keys

This adds an 'eckey' command to generate an EC key, with an optional
curve name argument, with P-256 as default.

For the 'selfsigned' command, it adds an 'ec' algorithm argument to the
'-newkey' option, and a '-pkeyopt ec_paramgen_curve:<curvename>' option,
mirroring the way openssl specifies the curve name.

Notice that curve names are not necessarily the same in mbedtls and
openssl.  In particular, secp256r1 works for mbedtls, but openssl uses
prime256v1 instead. px5g uses mbedtls, but short NIST curve names P-256
and P-384 are specifically supported.

Package size increased by about 900 bytes (arm).

Signed-off-by: Eneas U de Queiroz <cotequeiroz@gmail.com>