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/ls1021aqds_nand_defconfig b/configs/ls1021aqds_nand_defconfig
index 70a5193..bdb66d0 100644
--- a/configs/ls1021aqds_nand_defconfig
+++ b/configs/ls1021aqds_nand_defconfig
@@ -50,6 +50,7 @@
 CONFIG_SPL_NAND_SUPPORT=y
 CONFIG_SPL_WATCHDOG=y
 CONFIG_HUSH_PARSER=y
+CONFIG_SYS_CBSIZE=256
 CONFIG_SYS_PBSIZE=276
 CONFIG_CMD_BOOTZ=y
 CONFIG_CMD_IMLS=y