Convert CONFIG_SYS_CBSIZE to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_CBSIZE

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/iot_devkit_defconfig b/configs/iot_devkit_defconfig
index d415cc3..1380a6a 100644
--- a/configs/iot_devkit_defconfig
+++ b/configs/iot_devkit_defconfig
@@ -14,6 +14,7 @@
 CONFIG_LOCALVERSION="-iotdk-1.0"
 # CONFIG_ARCH_FIXUP_FDT_MEMORY is not set
 CONFIG_SYS_PROMPT="IoTDK# "
+CONFIG_SYS_CBSIZE=256
 CONFIG_SYS_PBSIZE=280
 # CONFIG_CMD_BOOTD is not set
 # CONFIG_CMD_BOOTM is not set