WIP: main/towboot-flasher: new aport

Needs extensive testing and somebody to write the SPI implementation 😛

Let's start flashing Tow-Boot on all devices that are supported by it. Like on the PPP it improves the booting experience by standardizing it, and allows us to use UEFI booting in the future. It will also allow us to eventually drop our own u-boot fork (the device install instructions have to be changed to include a note that the user should install Tow-Boot first if they haven't already). The goal is to drop this package in due time when enough time has passed and enough installations have Tow-Boot installed.