mmc: sandbox: rename CONFIG, fix dependency, and use it in Makefile

[1] Rename CONFIG_SANDBOX_MMC to CONFIG_MMC_SANDBOX for consistency
    I want all MMC driver options prefixed with CONFIG_MMC_.

[2] Fix dependency
    Add necessary depends on to avoid compile error.
    Instead "depends on MMC" is unneeded because this config entry
    resides inside of "if MMC".

[3] Currently, this config symbol is not referenced at all.
    Use it to enable/disable the driver in Makefile.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>