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/sama5d4ek_nandflash_defconfig b/configs/sama5d4ek_nandflash_defconfig
index 7e04a20..4f016bc 100644
--- a/configs/sama5d4ek_nandflash_defconfig
+++ b/configs/sama5d4ek_nandflash_defconfig
@@ -35,6 +35,7 @@
 CONFIG_SPL_NAND_DRIVERS=y
 CONFIG_SPL_NAND_BASE=y
 CONFIG_HUSH_PARSER=y
+CONFIG_SYS_CBSIZE=256
 CONFIG_SYS_PBSIZE=276
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_IMI is not set