Currently it just calls random().

Signed-off-by: Marty Connor <mdc@etherboot.org>

This fixes an issue where passing a length as a compound expression
(e.g. using `hdrlen + datalen') would trigger compiler warnings and
potentially precedence-related errors.

Signed-off-by: Marty Connor <mdc@etherboot.org>

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

It is a programming error, not a runtime error, if we attempt to use
block ciphers with an incorrect blocksize, so use an assert() rather
than an error status return.

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.