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/cortina_presidio-asic-pnand_defconfig b/configs/cortina_presidio-asic-pnand_defconfig
index b8b8684..867db8c 100644
--- a/configs/cortina_presidio-asic-pnand_defconfig
+++ b/configs/cortina_presidio-asic-pnand_defconfig
@@ -21,6 +21,7 @@
 CONFIG_LAST_STAGE_INIT=y
 CONFIG_SYS_PROMPT="G3#"
 CONFIG_SYS_MAXARGS=64
+CONFIG_SYS_CBSIZE=256
 CONFIG_SYS_PBSIZE=276
 CONFIG_CMD_MTD=y
 CONFIG_CMD_WDT=y