DPP: Build bootstrapping key DER encoding using custom routine

DPP: Build bootstrapping key DER encoding using custom routine

While the OpenSSL version of i2d_EC_PUBKEY() seemed to be able to use
the POINT_CONVERSION_COMPRESSED setting on the EC key, that did not seem
to work with BoringSSL. Since this is not exactly robust design, replace
use of i2d_EC_PUBKEY() with a custom routine that enforces the DPP rules
on SubjectPublicKeyInfo (compressed format of the public key,
ecPublicKey OID, parameters present and indicating the curve by OID).

Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>