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/M5253DEMO_defconfig b/configs/M5253DEMO_defconfig
index 6f69aca..b8a0cb4 100644
--- a/configs/M5253DEMO_defconfig
+++ b/configs/M5253DEMO_defconfig
@@ -13,6 +13,7 @@
 # CONFIG_DISPLAY_BOARDINFO is not set
 # CONFIG_CMDLINE_EDITING is not set
 # CONFIG_AUTO_COMPLETE is not set
+CONFIG_SYS_PBSIZE=276
 CONFIG_CMD_IMLS=y
 CONFIG_CMD_IDE=y
 # CONFIG_CMD_SETEXPR is not set