Convert CONFIG_CMD_MTDPARTS_SPREAD to Kconfig

This converts the following to Kconfig:
   CONFIG_CMD_MTDPARTS_SPREAD

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
[trini: Make this a bool not a string]
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/sandbox_defconfig b/configs/sandbox_defconfig
index fd0b952..698ced0 100644
--- a/configs/sandbox_defconfig
+++ b/configs/sandbox_defconfig
@@ -61,6 +61,7 @@
 CONFIG_CMD_CBFS=y
 CONFIG_CMD_CRAMFS=y
 CONFIG_CMD_EXT4_WRITE=y
+CONFIG_CMD_MTDPARTS=y
 CONFIG_MAC_PARTITION=y
 CONFIG_AMIGA_PARTITION=y
 CONFIG_OF_CONTROL=y