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>