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/mscc_servalt_defconfig b/configs/mscc_servalt_defconfig
index 018c618..cf1c4db 100644
--- a/configs/mscc_servalt_defconfig
+++ b/configs/mscc_servalt_defconfig
@@ -22,6 +22,7 @@
 CONFIG_DISPLAY_CPUINFO=y
 CONFIG_BOARD_EARLY_INIT_R=y
 CONFIG_SYS_PROMPT="servalt # "
+CONFIG_SYS_CBSIZE=256
 CONFIG_SYS_PBSIZE=283
 # CONFIG_CMD_BDI is not set
 # CONFIG_CMD_CONSOLE is not set