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_aemv8a_juno_defconfig b/configs/vexpress_aemv8a_juno_defconfig
index 0aca0fa..688cecb 100644
--- a/configs/vexpress_aemv8a_juno_defconfig
+++ b/configs/vexpress_aemv8a_juno_defconfig
@@ -15,6 +15,7 @@
 # CONFIG_DISPLAY_BOARDINFO is not set
 CONFIG_SYS_PROMPT="VExpress64# "
 CONFIG_SYS_MAXARGS=64
+CONFIG_SYS_CBSIZE=512
 CONFIG_SYS_PBSIZE=541
 CONFIG_CMD_MEMTEST=y
 CONFIG_CMD_ARMFLASH=y