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_sdcard_qspi_defconfig b/configs/ls1021aqds_sdcard_qspi_defconfig
index 605bc23..230f147 100644
--- a/configs/ls1021aqds_sdcard_qspi_defconfig
+++ b/configs/ls1021aqds_sdcard_qspi_defconfig
@@ -47,6 +47,7 @@
 CONFIG_SPL_MPC8XXX_INIT_DDR=y
 CONFIG_SPL_WATCHDOG=y
 CONFIG_HUSH_PARSER=y
+CONFIG_SYS_CBSIZE=256
 CONFIG_SYS_PBSIZE=276
 CONFIG_CMD_BOOTZ=y
 CONFIG_CMD_GREPENV=y