suit_create_es256_public_key should include key length

[hannestschofenig]

Currently, the suit_create_es256_public_key() function assumes ECC keys based on NIST P256r1. We should make this function more generic to also use other key sizes.

The function can be found in https://github.com/yuichitk/libcsuit/blob/master/src/suit_cose.c

[hisanak]

I created 1 commit to solve this issue.
In addition to this, I realized we need another commits like in PR #12 to solve compile errors raised by my definition changes.