Add FILE_LICENCE declarations to almost all files that make up the
various standard builds of gPXE.

The various types of cryptographic algorithm are fundamentally
different, and it was probably a mistake to try to handle them via a
single common type.

pubkey_algorithm is a placeholder type for now.

code, rather than going through the digest layer.

Removed a spurious line of constants.

size-efficient, and will probably be rewritten.

The md5.c from MatrixSSL is too ugly to consider importing.