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/km_kirkwood_defconfig b/configs/km_kirkwood_defconfig
index bb96558..596856f 100644
--- a/configs/km_kirkwood_defconfig
+++ b/configs/km_kirkwood_defconfig
@@ -23,6 +23,7 @@
 CONFIG_RESET_PHY_R=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_MAXARGS=32
+CONFIG_SYS_CBSIZE=512
 CONFIG_SYS_PBSIZE=532
 # CONFIG_BOOTM_NETBSD is not set
 # CONFIG_BOOTM_PLAN9 is not set