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-imx6_defconfig b/configs/pico-imx6_defconfig
index 816f904..6887ee9 100644
--- a/configs/pico-imx6_defconfig
+++ b/configs/pico-imx6_defconfig
@@ -34,6 +34,7 @@
 CONFIG_SPL_USB_SDP_SUPPORT=y
 CONFIG_SPL_WATCHDOG=y
 CONFIG_SYS_MAXARGS=32
+CONFIG_SYS_PBSIZE=532
 CONFIG_CMD_BOOTMENU=y
 CONFIG_CMD_SPL=y
 CONFIG_CMD_SPL_WRITE_SIZE=0x20000