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/r2dplus_defconfig b/configs/r2dplus_defconfig
index 4061708..edb1591 100644
--- a/configs/r2dplus_defconfig
+++ b/configs/r2dplus_defconfig
@@ -17,6 +17,7 @@
 CONFIG_PREBOOT="pci enum"
 CONFIG_DISPLAY_BOARDINFO=y
 CONFIG_HUSH_PARSER=y
+CONFIG_SYS_PBSIZE=256
 CONFIG_CMD_IMLS=y
 CONFIG_CMD_DM=y
 CONFIG_CMD_IDE=y