Feature Request: allow custom linker scripts

Because my project is solar powered, the power to it is sometimes very "dirty." This is causing the bootloader to trigger (despite a bug I fixed). I want to just flash my code without any bootloader, but that means changing the linker script, which currently can only be done platform-wide.

It'd be great if I could include a linker script in my sketch like any other source file.

Thanks!

No file or test is named. Start by tracing how ArduinoCore-samd currently selects the platform-wide linker script and how sketches enter the build flow. Done should mean a sketch can provide its own linker script and be flashed without the bootloader, while preserving the existing platform-wide behavior.

Written by the indexing model from the issue text.