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/vexpress_ca9x4_defconfig b/configs/vexpress_ca9x4_defconfig
index 51cf3629..258337e 100644
--- a/configs/vexpress_ca9x4_defconfig
+++ b/configs/vexpress_ca9x4_defconfig
@@ -16,6 +16,7 @@
 CONFIG_DEFAULT_FDT_FILE="vexpress-v2p-ca9.dtb"
 # CONFIG_DISPLAY_CPUINFO is not set
 # CONFIG_DISPLAY_BOARDINFO is not set
+CONFIG_SYS_CBSIZE=512
 CONFIG_SYS_PBSIZE=532
 # CONFIG_CMD_CONSOLE is not set
 # CONFIG_CMD_BOOTD is not set