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/coreboot_defconfig b/configs/coreboot_defconfig
index b09f3f0..792d35e 100644
--- a/configs/coreboot_defconfig
+++ b/configs/coreboot_defconfig
@@ -19,6 +19,7 @@
 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