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_defconfig b/configs/display5_defconfig
index 0c47e83..2b92ed9 100644
--- a/configs/display5_defconfig
+++ b/configs/display5_defconfig
@@ -50,6 +50,7 @@
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="display5 > "
 CONFIG_SYS_MAXARGS=32
+CONFIG_SYS_CBSIZE=2048
 CONFIG_SYS_PBSIZE=2076
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_ELF is not set