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/kontron-sl-mx6ul_defconfig b/configs/kontron-sl-mx6ul_defconfig
index 2712d4b..1ba4d1f 100644
--- a/configs/kontron-sl-mx6ul_defconfig
+++ b/configs/kontron-sl-mx6ul_defconfig
@@ -32,6 +32,7 @@
 CONFIG_SPL_USB_GADGET=y
 CONFIG_SPL_WATCHDOG=y
 CONFIG_SYS_MAXARGS=32
+CONFIG_SYS_PBSIZE=532
 CONFIG_CMD_MEMTEST=y
 CONFIG_CMD_GPIO=y
 CONFIG_CMD_I2C=y