MMC: add a dedicated backend for MMC so that supported-opcode map works

MMC has been broken for a while since some of the vital opcodes
(GET_CONFIGURATION) and friends are not in the supported opcode
whitelist in bs_rdwr.c.
The real problem is that we have a bs_rdwr.c backend for SBC and we
piggyback on this SBC backend also for the MMC emulation.

Add defines for the missing MMC opcodes and create a new backend
dedicated for MMC that has the proper whitelist for MMC opcodes.
This new backend is actually part of bs_rdwr.c and shares all the
code with the SBC based backend.
However the new MMC backend will have a different name "mmc" to indicate
it is for the MMC emulation and it comes with a supported opcode
whitelist that is appropriate for MMC devices.

Signed-off-by: Ronnie Sahlberg <ronniesahlberg@gmail.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
DOCS: add an example dvd jukebox script

Add an example script that can be used to automatically set up a DVD jukebox
with a dvd drive and a medium changer that will export all your ISO images.

Use the mtx command to operate which media should be loaded.

Signed-off-by: Ronnie Sahlberg <ronniesahlberg@gmail.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>