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/pico-pi-imx6ul_defconfig b/configs/pico-pi-imx6ul_defconfig
index 6bc8d5d..2cc6d4f 100644
--- a/configs/pico-pi-imx6ul_defconfig
+++ b/configs/pico-pi-imx6ul_defconfig
@@ -30,6 +30,7 @@
 CONFIG_SPL_USB_GADGET=y
 CONFIG_SPL_USB_SDP_SUPPORT=y
 CONFIG_SYS_MAXARGS=32
+CONFIG_SYS_PBSIZE=532
 CONFIG_CMD_BOOTMENU=y
 CONFIG_CMD_MEMTEST=y
 CONFIG_CMD_DFU=y