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-emmc_defconfig b/configs/cortina_presidio-asic-emmc_defconfig
index 228215b..cb82636 100644
--- a/configs/cortina_presidio-asic-emmc_defconfig
+++ b/configs/cortina_presidio-asic-emmc_defconfig
@@ -19,6 +19,7 @@
 CONFIG_LAST_STAGE_INIT=y
 CONFIG_SYS_PROMPT="G3#"
 CONFIG_SYS_MAXARGS=64
+CONFIG_SYS_CBSIZE=256
 CONFIG_SYS_PBSIZE=276
 CONFIG_CMD_I2C=y
 CONFIG_CMD_MMC=y