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>
diff --git a/configs/sandbox_defconfig b/configs/sandbox_defconfig
index 877aa94..01f6f5d 100644
--- a/configs/sandbox_defconfig
+++ b/configs/sandbox_defconfig
@@ -94,7 +94,7 @@
 CONFIG_PWRSEQ=y
 CONFIG_SPL_PWRSEQ=y
 CONFIG_I2C_EEPROM=y
-CONFIG_SANDBOX_MMC=y
+CONFIG_MMC_SANDBOX=y
 CONFIG_SPI_FLASH_SANDBOX=y
 CONFIG_SPI_FLASH=y
 CONFIG_SPI_FLASH_ATMEL=y