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/seeed_npi_imx6ull_defconfig b/configs/seeed_npi_imx6ull_defconfig
index 95cb376..8fb5590 100644
--- a/configs/seeed_npi_imx6ull_defconfig
+++ b/configs/seeed_npi_imx6ull_defconfig
@@ -26,6 +26,7 @@
 CONFIG_SPL_USB_GADGET=y
 CONFIG_SPL_WATCHDOG=y
 CONFIG_SYS_MAXARGS=32
+CONFIG_SYS_PBSIZE=532
 CONFIG_CMD_MEMTEST=y
 CONFIG_CMD_DM=y
 CONFIG_CMD_GPIO=y