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/ap121_defconfig b/configs/ap121_defconfig
index 1ae6190..0baa4c8 100644
--- a/configs/ap121_defconfig
+++ b/configs/ap121_defconfig
@@ -21,6 +21,7 @@
 CONFIG_DISPLAY_CPUINFO=y
 CONFIG_BOARD_EARLY_INIT_F=y
 CONFIG_SYS_PROMPT="ap121 # "
+CONFIG_SYS_CBSIZE=256
 CONFIG_SYS_PBSIZE=281
 # CONFIG_CMD_BDI is not set
 # CONFIG_CMD_CONSOLE is not set