Fix bug in rnd_pseudo_rnd() test helper function

Only the first 4 bytes of the output were set, the rest was untouched.
Remove bias in EC private key generation
Adapt ECDSA tests to new ecp_gen_keypair()
Fix unchecked error codes in ecp_gen_keypair()
Fix theoretical compliance issue in ECDSA

The issue would happen for curves whose bitlength is not a multiple of eight
(the only case is NIST P-521) with hashes that are longer than the bitlength
of the curve: since the wides hash is 512 bits long, this can't happen.
Fixing however as a matter of principle and readability.
Add minified HMAC_DRBG for deterministic ECDSA
Add ecdsa_sign_det() with test vectors
Little HMAC_DRBG refactoring
Add ecdsa_write_signature_det()
Add fallback for md_alg == NONE to ecdsa_sign_det()
Use deterministic ECDSA in the PK layer
Document dependency of ECDSA_DETERMINISTIC on MD_C
Fix bignum ASM format for X86-64
Fix bignum's ASM format for MIPS
Fix BN ASM for PowerPC
Fix asm format for MC68020
Fix asm format for microblaze
Fix asm format for tricore
Fix asm format for alpha
Add support for secp256k1 arithmetic