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/durian_defconfig b/configs/durian_defconfig
index 2ebb691..c4eb392 100644
--- a/configs/durian_defconfig
+++ b/configs/durian_defconfig
@@ -18,6 +18,7 @@
 # CONFIG_DISPLAY_BOARDINFO is not set
 CONFIG_LAST_STAGE_INIT=y
 CONFIG_SYS_PROMPT="durian#"
+CONFIG_SYS_CBSIZE=256
 CONFIG_SYS_PBSIZE=280
 # CONFIG_CMD_LZMADEC is not set
 # CONFIG_CMD_UNZIP is not set