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/imx8mn_venice_defconfig b/configs/imx8mn_venice_defconfig
index 4b8dde8..7c23351 100644
--- a/configs/imx8mn_venice_defconfig
+++ b/configs/imx8mn_venice_defconfig
@@ -39,6 +39,7 @@
 CONFIG_SPL_WATCHDOG=y
 CONFIG_SYS_PROMPT="u-boot=> "
 CONFIG_SYS_MAXARGS=64
+CONFIG_SYS_CBSIZE=2048
 CONFIG_SYS_PBSIZE=2074
 # CONFIG_CMD_EXPORTENV is not set
 # CONFIG_CMD_IMPORTENV is not set