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/at91sam9g20ek_2mmc_defconfig b/configs/at91sam9g20ek_2mmc_defconfig
index 93018b72..90bd151 100644
--- a/configs/at91sam9g20ek_2mmc_defconfig
+++ b/configs/at91sam9g20ek_2mmc_defconfig
@@ -26,6 +26,7 @@
 CONFIG_BOOTCOMMAND="fatload mmc 0:1 0x22000000 uImage; bootm"
 # CONFIG_DISPLAY_BOARDINFO is not set
 CONFIG_SYS_PROMPT="U-Boot> "
+CONFIG_SYS_CBSIZE=256
 CONFIG_SYS_PBSIZE=281
 # CONFIG_CMD_BDI is not set
 CONFIG_CMD_BOOTZ=y