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/SBx81LIFKW_defconfig b/configs/SBx81LIFKW_defconfig
index fb12b9b..3c77bdc 100644
--- a/configs/SBx81LIFKW_defconfig
+++ b/configs/SBx81LIFKW_defconfig
@@ -20,6 +20,7 @@
 CONFIG_MISC_INIT_R=y
 CONFIG_RESET_PHY_R=y
 CONFIG_HUSH_PARSER=y
+CONFIG_SYS_CBSIZE=256
 CONFIG_SYS_PBSIZE=276
 CONFIG_CMD_DM=y
 # CONFIG_CMD_FLASH is not set