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/stm32h750-art-pi_defconfig b/configs/stm32h750-art-pi_defconfig
index e4403a2..932c78e 100644
--- a/configs/stm32h750-art-pi_defconfig
+++ b/configs/stm32h750-art-pi_defconfig
@@ -23,6 +23,7 @@
 CONFIG_BOARD_EARLY_INIT_F=y
 CONFIG_BOARD_LATE_INIT=y
 CONFIG_SYS_PROMPT="U-Boot > "
+CONFIG_SYS_CBSIZE=256
 CONFIG_SYS_PBSIZE=282
 CONFIG_CMD_GPT=y
 CONFIG_CMD_MMC=y