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/sama5d3_xplained_mmc_defconfig b/configs/sama5d3_xplained_mmc_defconfig
index a477d23..86724e4 100644
--- a/configs/sama5d3_xplained_mmc_defconfig
+++ b/configs/sama5d3_xplained_mmc_defconfig
@@ -33,6 +33,7 @@
 CONFIG_BOOTCOMMAND="if test ! -n ${dtb_name}; then setenv dtb_name at91-${board_name}.dtb; fi; fatload mmc 0:1 0x21000000 ${dtb_name}; fatload mmc 0:1 0x22000000 zImage; bootz 0x22000000 - 0x21000000"
 # CONFIG_DISPLAY_BOARDINFO is not set
 CONFIG_HUSH_PARSER=y
+CONFIG_SYS_CBSIZE=256
 CONFIG_SYS_PBSIZE=276
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_FLASH is not set