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/display5_factory_defconfig b/configs/display5_factory_defconfig
index cb27169..9c3965c 100644
--- a/configs/display5_factory_defconfig
+++ b/configs/display5_factory_defconfig
@@ -48,6 +48,7 @@
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="display5 factory > "
 CONFIG_SYS_MAXARGS=32
+CONFIG_SYS_CBSIZE=2048
 CONFIG_SYS_PBSIZE=2084
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_ELF is not set