Uses build.rs in board directories to set the linker options locally.
The file .cargo/config can't be used for that, as in a workspace the
local .cargo/config is ignored if the build is triggered from the
workspace root.
Current version of probe-run is 0.3.3, which uses defmt 0.3.2.
With a firmware using defmt 0.2, this causes the following error
message:
```
Error: defmt wire format version mismatch: firmware is using 0.2, `probe-run` supports 3
suggestion: `cargo install` a different version of `probe-run` that supports defmt 0.2
```
Therefore, upgrade defmt dependency, and also fix the linker script
in .cargo/config.
Sorry this is a large commit :(
This adds support for input pins, including pulling them high or low.
It also adds two examples: the start of a classic blinky LED example, and an example for reading input.
