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/imx8qm_mek_defconfig b/configs/imx8qm_mek_defconfig
index cdd69cf..1bc0a9d 100644
--- a/configs/imx8qm_mek_defconfig
+++ b/configs/imx8qm_mek_defconfig
@@ -35,6 +35,7 @@
 CONFIG_SPL_POWER_DOMAIN=y
 CONFIG_SPL_WATCHDOG=y
 CONFIG_HUSH_PARSER=y
+CONFIG_SYS_CBSIZE=256
 CONFIG_SYS_PBSIZE=276
 CONFIG_CMD_CPU=y
 # CONFIG_BOOTM_NETBSD is not set