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/vf610twr_nand_defconfig b/configs/vf610twr_nand_defconfig
index 8ab5157..f6ed47f 100644
--- a/configs/vf610twr_nand_defconfig
+++ b/configs/vf610twr_nand_defconfig
@@ -25,6 +25,7 @@
 CONFIG_HUSH_PARSER=y
 # CONFIG_CMDLINE_EDITING is not set
 # CONFIG_AUTO_COMPLETE is not set
+CONFIG_SYS_CBSIZE=256
 CONFIG_SYS_PBSIZE=276
 CONFIG_CMD_BOOTZ=y
 CONFIG_CMD_MEMTEST=y