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_serval_defconfig b/configs/mscc_serval_defconfig
index f034fd6..a56c5eb 100644
--- a/configs/mscc_serval_defconfig
+++ b/configs/mscc_serval_defconfig
@@ -23,6 +23,7 @@
 CONFIG_DISPLAY_CPUINFO=y
 CONFIG_BOARD_EARLY_INIT_R=y
 CONFIG_SYS_PROMPT="serval # "
+CONFIG_SYS_CBSIZE=256
 CONFIG_SYS_PBSIZE=282
 # CONFIG_CMD_BDI is not set
 # CONFIG_CMD_CONSOLE is not set