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/poleg_evb_defconfig b/configs/poleg_evb_defconfig
index 25a774d..739bfcf 100644
--- a/configs/poleg_evb_defconfig
+++ b/configs/poleg_evb_defconfig
@@ -17,6 +17,7 @@
 CONFIG_BOOTCOMMAND="run common_bootargs; run romboot"
 CONFIG_SYS_PROMPT="U-Boot>"
 CONFIG_SYS_MAXARGS=32
+CONFIG_SYS_CBSIZE=256
 CONFIG_SYS_PBSIZE=280
 CONFIG_CMD_GPIO=y
 CONFIG_CMD_SPI=y