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/ls1021atwr_nor_defconfig b/configs/ls1021atwr_nor_defconfig
index 429cada..e80b793 100644
--- a/configs/ls1021atwr_nor_defconfig
+++ b/configs/ls1021atwr_nor_defconfig
@@ -31,6 +31,7 @@
 CONFIG_ARCH_MISC_INIT=y
 CONFIG_MISC_INIT_R=y
 CONFIG_ID_EEPROM=y
+CONFIG_SYS_CBSIZE=256
 CONFIG_SYS_PBSIZE=276
 CONFIG_CMD_IMLS=y
 CONFIG_CMD_GREPENV=y