DPP: Use a helper function to DER encode bootstrapping key

DPP: Use a helper function to DER encode bootstrapping key

This routine was previously implemented twice using i2d_EC_PUBKEY().
There is no need to duplicate that implementation and especially since
it looks like this implementation needs to be replaced for BoringSSL,
start by using a shared helper function for both locations so that there
is only a single place that uses i2d_EC_PUBKEY() to build the special
DPP bootstrapping key DER encoding.

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