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/thunderx_88xx_defconfig b/configs/thunderx_88xx_defconfig
index bb87c89..ee7818d 100644
--- a/configs/thunderx_88xx_defconfig
+++ b/configs/thunderx_88xx_defconfig
@@ -22,6 +22,7 @@
 # CONFIG_AUTO_COMPLETE is not set
 CONFIG_SYS_PROMPT="ThunderX_88XX> "
 CONFIG_SYS_MAXARGS=64
+CONFIG_SYS_CBSIZE=512
 CONFIG_SYS_PBSIZE=544
 # CONFIG_CMD_EXPORTENV is not set
 # CONFIG_CMD_IMPORTENV is not set