Improve customization of linker scripts

[mattnite]

Right now in raspberrypi-rp2040 we've had to hardcode the linker script to allow a boot2 section, time_critical section, and have all of sram executable so that some of it may be executed by the processor. Linker scripts can be pretty complex, but maybe we can reduce the feature to only what's typically required in embedded systems. If users are already at the point where they need very specific features or linker script layouts then it's already easy enough for them to write their own.