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/usb_a9263_dataflash_defconfig b/configs/usb_a9263_dataflash_defconfig
index 65f6a5e..e6dcd09 100644
--- a/configs/usb_a9263_dataflash_defconfig
+++ b/configs/usb_a9263_dataflash_defconfig
@@ -20,6 +20,7 @@
 # CONFIG_DISPLAY_BOARDINFO is not set
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="U-Boot> "
+CONFIG_SYS_PBSIZE=281
 # CONFIG_CMD_BDI is not set
 # CONFIG_CMD_IMI is not set
 # CONFIG_CMD_LOADB is not set