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/smartweb_defconfig b/configs/smartweb_defconfig
index 717baa2..a9f90e9 100644
--- a/configs/smartweb_defconfig
+++ b/configs/smartweb_defconfig
@@ -39,6 +39,7 @@
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="U-Boot# "
 CONFIG_SYS_MAXARGS=32
+CONFIG_SYS_CBSIZE=512
 CONFIG_SYS_PBSIZE=537
 # CONFIG_CMD_IMI is not set
 CONFIG_CMD_DFU=y