wolfssl: Fixes, code style changes and some refactorings

The main fixes are

 * the generation of fingerprints for RSA, ECDSA, and EdDSA
 * the encoding of ECDSA private keys
 * calculating p and q for RSA private keys
 * deriving the public key for raw Ed25519 private keys

Also, instead of numeric literals for buffer lengths ASN.1 related
constants are used.

conf/plugins/wolfssl.opt

@@ -0,0 +1,2 @@
+charon.plugins.wolfssl.fips_mode = no
+	Enable to prevent loading the plugin if wolfSSL is not in FIPS mode.