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/drivers/mmc/Makefile b/drivers/mmc/Makefile
index 6af7f79..2747deb 100644
--- a/drivers/mmc/Makefile
+++ b/drivers/mmc/Makefile
@@ -40,11 +40,7 @@
 obj-$(CONFIG_PXA_MMC_GENERIC) += pxa_mmc_gen.o
 obj-$(CONFIG_SUPPORT_EMMC_RPMB) += rpmb.o
 obj-$(CONFIG_S3C_SDI) += s3c_sdi.o
-ifdef CONFIG_BLK
-ifdef CONFIG_GENERIC_MMC
-obj-$(CONFIG_SANDBOX) += sandbox_mmc.o
-endif
-endif
+obj-$(CONFIG_MMC_SANDBOX)		+= sandbox_mmc.o
 obj-$(CONFIG_SH_MMCIF) += sh_mmcif.o
 obj-$(CONFIG_SH_SDHI) += sh_sdhi.o
 obj-$(CONFIG_ZYNQ_SDHCI) += zynq_sdhci.o