libmediaart: rename options to have proper prefix

Options should be prefixed by BR2_PACKAGE_LIBMEDIAART and not
BR2_PACKAGE_MEDIAART, but package was using both prefixes.

This was found as default symbol was defined as
BR2_PACKAGE_LIBMEDIAART_BACKEND_NONE (correct prefix), but symbol
was actually BR2_PACKAGE_MEDIAART_BACKEND_NONE).

This commit therefore renames the incorrectly named options, and adds
Config.in.legacy handling. Since the options are part of a choice, the
legacy handling cannot select the new options, and is only here to
inform the user.

Fixes: c443830a libmediaart: new package

Signed-off-by: Petr Vorel <petr.vorel@gmail.com>
[Thomas: improve commit log, add Config.in.legacy handling]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>