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/chromebox_panther_defconfig b/configs/chromebox_panther_defconfig
index 363b5f3..c909d31 100644
--- a/configs/chromebox_panther_defconfig
+++ b/configs/chromebox_panther_defconfig
@@ -24,6 +24,7 @@
 CONFIG_LAST_STAGE_INIT=y
 CONFIG_MISC_INIT_R=y
 CONFIG_HUSH_PARSER=y
+CONFIG_SYS_PBSIZE=532
 CONFIG_CMD_GPIO=y
 CONFIG_CMD_PART=y
 CONFIG_CMD_SPI=y