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/bcm968360bg_ram_defconfig b/configs/bcm968360bg_ram_defconfig
index 489e1b1..9d1202c 100644
--- a/configs/bcm968360bg_ram_defconfig
+++ b/configs/bcm968360bg_ram_defconfig
@@ -19,6 +19,7 @@
 CONFIG_DISPLAY_BOARDINFO_LATE=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_MAXARGS=24
+CONFIG_SYS_CBSIZE=256
 CONFIG_SYS_PBSIZE=276
 CONFIG_CMD_GPIO=y
 CONFIG_CMD_MTD=y