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/bcm96753ref_ram_defconfig b/configs/bcm96753ref_ram_defconfig
index 8f69f86..ac710f4 100644
--- a/configs/bcm96753ref_ram_defconfig
+++ b/configs/bcm96753ref_ram_defconfig
@@ -24,6 +24,7 @@
 CONFIG_SYS_CONSOLE_IS_IN_ENV=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_MAXARGS=24
+CONFIG_SYS_CBSIZE=256
 CONFIG_SYS_PBSIZE=276
 # CONFIG_CMD_BOOTD is not set
 # CONFIG_BOOTM_NETBSD is not set