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/comtrend_ar5387un_ram_defconfig b/configs/comtrend_ar5387un_ram_defconfig
index d9a97c5..67fa622 100644
--- a/configs/comtrend_ar5387un_ram_defconfig
+++ b/configs/comtrend_ar5387un_ram_defconfig
@@ -20,6 +20,7 @@
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="AR-5387un # "
 CONFIG_SYS_MAXARGS=24
+CONFIG_SYS_CBSIZE=512
 CONFIG_SYS_PBSIZE=541
 CONFIG_CMD_CPU=y
 CONFIG_CMD_LICENSE=y