tools: add USB debugg helper scripts (spcon, spmount, spoff)

These tools can save you the hassle of plugging and unplugging the USB
cable repeatedly when doing build/test cycles. Make sure to edit the
device files and numbers in the scripts to match your setup.
libgame: adbg: make buffer stuff static

So it won't show up in the documentation.
libgame: usbdbg: improve documentation (slightly)
libgame: main.dox: fix typo