Convert CONFIG_MTD_PARTITIONS et al to Kconfig

This converts the following to Kconfig:
   CONFIG_MTD_PARTITIONS
   CONFIG_MTD_DEVICE

Signed-off-by: Adam Ford <aford173@gmail.com>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index 2fa0829..c5a7a3c 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -1664,6 +1664,7 @@
 
 config CMD_MTDPARTS
 	bool "MTD partition support"
+	select MTD_DEVICE if (CMD_NAND || NAND)
 	help
 	  MTD partition support