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/coreboot64_defconfig b/configs/coreboot64_defconfig
index 3427848..a61f869 100644
--- a/configs/coreboot64_defconfig
+++ b/configs/coreboot64_defconfig
@@ -21,6 +21,7 @@
 CONFIG_DISPLAY_BOARDINFO_LATE=y
 CONFIG_LAST_STAGE_INIT=y
 CONFIG_HUSH_PARSER=y
+CONFIG_SYS_PBSIZE=532
 CONFIG_CMD_IDE=y
 CONFIG_CMD_MMC=y
 CONFIG_CMD_PART=y