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/configs/vining_2000_defconfig b/configs/vining_2000_defconfig
index 1d28b2f..8cc030b 100644
--- a/configs/vining_2000_defconfig
+++ b/configs/vining_2000_defconfig
@@ -30,6 +30,7 @@
 CONFIG_CMD_FS_GENERIC=y
 CONFIG_EFI_PARTITION=y
 CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG=y
+CONFIG_SUPPORT_EMMC_RPMB=y
 CONFIG_FSL_ESDHC=y
 CONFIG_PHYLIB=y
 CONFIG_PCI=y