The current API for Base16 (and Base64) encoding requires the caller
to always provide sufficient buffer space.  This prevents the use of
the generic encoding/decoding functionality in some situations, such
as in formatting the hex setting types.

Implement a generic hex_encode() (based on the existing
format_hex_setting()), implement base16_encode() and base16_decode()
in terms of the more generic hex_encode() and hex_decode(), and update
all callers to provide the additional buffer length parameter.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

Relicense files for which I am the sole author (as identified by
util/relicense.pl).

Signed-off-by: Michael Brown <mcb30@ipxe.org>

Provide a generic hex_decode() routine which can be shared between the
Base16 code and the "hex" and "hexhyp" settings parsers.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

Base16 encoding is currently implemented in both iSCSI and SRP.

Signed-off-by: Michael Brown <mcb30@ipxe.org>