Convert CONFIG_SUPPORT_EMMC_RPMB to Kconfig

Convert CONFIG_SUPPORT_EMMC_RPMB to Kconfig. Split the command handling
from the underlying support and expose this through CMD_MMC_RPMB.

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index 38406fc..036bad8 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -816,6 +816,13 @@
 	help
 	  MMC memory mapped support.
 
+config CMD_MMC_RPMB
+	bool "Enable support for RPMB in the mmc command"
+	depends on CMD_MMC
+	help
+	  Enable the commands for reading, writing and programming the
+	  key for the Replay Protection Memory Block partition in eMMC.
+
 config CMD_NAND
 	bool "nand"
 	default y if NAND_SUNXI