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_aemv8r_defconfig b/configs/vexpress_aemv8r_defconfig
index 928b64b..2cc28d6 100644
--- a/configs/vexpress_aemv8r_defconfig
+++ b/configs/vexpress_aemv8r_defconfig
@@ -12,6 +12,7 @@
 # CONFIG_DISPLAY_CPUINFO is not set
 CONFIG_SYS_PROMPT="VExpress64# "
 CONFIG_SYS_MAXARGS=64
+CONFIG_SYS_CBSIZE=512
 CONFIG_SYS_PBSIZE=541
 # CONFIG_MMC is not set
 CONFIG_VIRTIO_MMIO=y