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/tb100_defconfig b/configs/tb100_defconfig
index 0fe5379..d9d6726 100644
--- a/configs/tb100_defconfig
+++ b/configs/tb100_defconfig
@@ -11,6 +11,7 @@
 CONFIG_USE_BOOTARGS=y
 CONFIG_BOOTARGS="console=ttyS0,115200n8"
 CONFIG_SYS_PROMPT="[tb100]:~# "
+CONFIG_SYS_CBSIZE=256
 CONFIG_SYS_PBSIZE=284
 # CONFIG_CMD_SETEXPR is not set
 CONFIG_CMD_DHCP=y