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/kmcent2_defconfig b/configs/kmcent2_defconfig
index a6b5015..17e899a 100644
--- a/configs/kmcent2_defconfig
+++ b/configs/kmcent2_defconfig
@@ -24,6 +24,7 @@
 CONFIG_LAST_STAGE_INIT=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_MAXARGS=32
+CONFIG_SYS_CBSIZE=512
 CONFIG_SYS_PBSIZE=532
 CONFIG_CMD_DM=y
 CONFIG_CMD_I2C=y