upload.tool not used in custom boards.txt?
From https://github.com/bigjosh/Move38-Arduino-Platform#programmers:


  Since there is no bootloader in a tile, all code must be programmed rather than downloaded.


When you do an "Upload Using Programmer", the tool is specified in the {programmerID}.program.tool property in programmers.txt. The {boardID}.upload.tool value is only used for a standard upload.

Since programmers.txt is not in your hardware package, this makes it very inconvenient to customize. What's worse is that the platform.txt associated with the selected programmer is used, rather than the one in your hardware package, for "Upload Using Programmer". For this reason, some popular hardware packages include their own copies of all standard programmers. The downside of this is it really clutters up the Tools > Programmer menu if you have multiple of these packages installed.