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/imx8mp_rsb3720a1_6G_defconfig b/configs/imx8mp_rsb3720a1_6G_defconfig
index 1d89848..3b0e02e 100644
--- a/configs/imx8mp_rsb3720a1_6G_defconfig
+++ b/configs/imx8mp_rsb3720a1_6G_defconfig
@@ -44,6 +44,7 @@
 CONFIG_SPL_WATCHDOG=y
 CONFIG_SYS_PROMPT="u-boot=> "
 CONFIG_SYS_MAXARGS=64
+CONFIG_SYS_CBSIZE=2048
 CONFIG_SYS_PBSIZE=2074
 CONFIG_CMD_BOOTEFI_SELFTEST=y
 CONFIG_CMD_ERASEENV=y