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/netgear_dgnd3700v2_ram_defconfig b/configs/netgear_dgnd3700v2_ram_defconfig
index d305459..e1f9cee 100644
--- a/configs/netgear_dgnd3700v2_ram_defconfig
+++ b/configs/netgear_dgnd3700v2_ram_defconfig
@@ -21,6 +21,7 @@
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="DGND3700v2 # "
 CONFIG_SYS_MAXARGS=24
+CONFIG_SYS_CBSIZE=512
 CONFIG_SYS_PBSIZE=542
 CONFIG_CMD_CPU=y
 CONFIG_CMD_LICENSE=y