tftpboot: compute dtbSize *after* changing dtb

Adding the reserved-memory node to the dtb can cause it to grow
by enough that it needs an extra page - this will overlap the start
of the kernel image if we calculate offsets based on the original size

Reported-by: sinavir
Authored-by: sinavir

modules/outputs/tftpboot.nix

@@ -74,8 +74,6 @@ in {
             rootfsSize=$(binsize64k ${o.rootfs} )
             rootfsSize=$(($rootfsSize + ${toString cfg.freeSpaceBytes} ))
             dtbStart=$(($rootfsStart + $rootfsSize))
-            dtbSize=$(binsize ${o.dtb} )
-            imageStart=$(($dtbStart + $dtbSize))
             imageSize=$(binsize ${image})
 
             ln -s ${o.manifest} manifest
@@ -103,6 +101,9 @@ in {
             fdtput -p -t s dtb /reserved-memory/$node compatible phram
             fdtput -p -t lx dtb /reserved-memory/$node reg $ac_prefix $(hex $rootfsStart) $sz_prefix $(hex $rootfsSize)
 
+            dtbSize=$(binsize ./dtb )
+            imageStart=$(($dtbStart + $dtbSize))
+
             cmd="liminix ${cmdline} mtdparts=phram0:''${rootfsSize}(rootfs) phram.phram=phram0,''${rootfsStart},''${rootfsSize},${toString config.hardware.flash.eraseBlockSize} root=/dev/mtdblock0";
             fdtput -t s dtb /chosen bootargs "$cmd"