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/stm32h743-disco_defconfig b/configs/stm32h743-disco_defconfig
index 9b60b3e..4989f2c 100644
--- a/configs/stm32h743-disco_defconfig
+++ b/configs/stm32h743-disco_defconfig
@@ -17,6 +17,7 @@
 CONFIG_DEFAULT_FDT_FILE="stm32h743i-disco"
 # CONFIG_DISPLAY_CPUINFO is not set
 CONFIG_SYS_PROMPT="U-Boot > "
+CONFIG_SYS_CBSIZE=256
 CONFIG_SYS_PBSIZE=282
 CONFIG_CMD_GPT=y
 CONFIG_CMD_MMC=y