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/deneb_defconfig b/configs/deneb_defconfig
index cd809b9..f02b10b 100644
--- a/configs/deneb_defconfig
+++ b/configs/deneb_defconfig
@@ -43,6 +43,7 @@
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="U-Boot# "
 CONFIG_SYS_MAXARGS=64
+CONFIG_SYS_CBSIZE=2048
 CONFIG_SYS_PBSIZE=2073
 CONFIG_CMD_CPU=y
 # CONFIG_BOOTM_NETBSD is not set