Convert CONFIG_SYS_PBSIZE to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_PBSIZE

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/mscc_luton_defconfig b/configs/mscc_luton_defconfig
index 502c8ef..99e85ec 100644
--- a/configs/mscc_luton_defconfig
+++ b/configs/mscc_luton_defconfig
@@ -28,6 +28,7 @@
 CONFIG_DISPLAY_CPUINFO=y
 CONFIG_BOARD_EARLY_INIT_R=y
 CONFIG_SYS_PROMPT="luton # "
+CONFIG_SYS_PBSIZE=281
 # CONFIG_CMD_BDI is not set
 # CONFIG_CMD_CONSOLE is not set
 # CONFIG_CMD_ELF is not set