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/mt8516_pumpkin_defconfig b/configs/mt8516_pumpkin_defconfig
index 5b10aeb..0425ffd 100644
--- a/configs/mt8516_pumpkin_defconfig
+++ b/configs/mt8516_pumpkin_defconfig
@@ -22,6 +22,7 @@
 # CONFIG_DISPLAY_BOARDINFO is not set
 CONFIG_BOARD_LATE_INIT=y
 CONFIG_HUSH_PARSER=y
+CONFIG_SYS_CBSIZE=256
 CONFIG_SYS_PBSIZE=276
 # CONFIG_CMD_BDI is not set
 # CONFIG_CMD_CONSOLE is not set