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/mt8518_ap1_emmc_defconfig b/configs/mt8518_ap1_emmc_defconfig
index da029ee..8a2ddab 100644
--- a/configs/mt8518_ap1_emmc_defconfig
+++ b/configs/mt8518_ap1_emmc_defconfig
@@ -15,6 +15,7 @@
 CONFIG_DEFAULT_FDT_FILE="mt8518-ap1-emmc.dtb"
 CONFIG_BOARD_LATE_INIT=y
 CONFIG_SYS_PROMPT="MT8518> "
+CONFIG_SYS_CBSIZE=256
 CONFIG_SYS_PBSIZE=281
 CONFIG_CMD_BOOTMENU=y
 CONFIG_CMD_MMC=y