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/verdin-imx8mm_defconfig b/configs/verdin-imx8mm_defconfig
index 4489213..ec47a0f 100644
--- a/configs/verdin-imx8mm_defconfig
+++ b/configs/verdin-imx8mm_defconfig
@@ -40,6 +40,7 @@
 CONFIG_SPL_WATCHDOG=y
 CONFIG_SYS_PROMPT="Verdin iMX8MM # "
 CONFIG_SYS_MAXARGS=64
+CONFIG_SYS_CBSIZE=2048
 CONFIG_SYS_PBSIZE=2081
 # CONFIG_BOOTM_NETBSD is not set
 CONFIG_CMD_ASKENV=y