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/meesc_dataflash_defconfig b/configs/meesc_dataflash_defconfig
index e69aae4..315c484 100644
--- a/configs/meesc_dataflash_defconfig
+++ b/configs/meesc_dataflash_defconfig
@@ -18,6 +18,7 @@
 CONFIG_BOARD_EARLY_INIT_F=y
 CONFIG_MISC_INIT_R=y
 CONFIG_HUSH_PARSER=y
+CONFIG_SYS_CBSIZE=512
 CONFIG_SYS_PBSIZE=532
 # CONFIG_CMD_BDI is not set
 CONFIG_CMD_BOOTZ=y