base16_encode() will fail to generate a terminating NUL if the length
of the raw data is zero, since the loop calling sprintf() will never
execute.

Fix by explicitly terminating the result with a NUL.

Reported-by: Marin Hannache <git@mareo.fr>
Debugged-by: Marin Hannache <git@mareo.fr>
Tested-by: Marin Hannache <git@mareo.fr>
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>

Suggested-by: Daniel P. Berrange <berrange@redhat.com>
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>