Set linker script from build.rs file

[jwnrt]

This has two advantages:

1. It adds a cargo:rerun-if-changed command to ensure we rebuild crates if the linker script changes.
2. The path to the linker script does not depend on the directory that Cargo is executed from, so cargo build can be done from anywhere.

[engdoreis]

Thanks @jwnrt.
How can we make it clear where the linker script is added? Maybe we could document on each Cargo.toml or print a println! on build.rs?

[jwnrt]

I have added a comment to the demo Cargo.toml files because println! in build.rs is hidden unless there's an error

[nbdd0121]

The path to the linker script does not depend on the directory that Cargo is executed from, so cargo build can be done from anywhere.

I think this one is not true, but rerun-if-changed is still a plus.

[jwnrt]

I think this one is not true, but rerun-if-changed is still a plus.

You're right - I was mistaken about the error message you get with cargo build. It was actually saying that there wasn't enough space to link correctly, not that the link was badly defined.

I've added an extra commit to enable optimizations for debug builds (level 1 for the crate and 2 for dependencies) so that cargo build just works.

[engdoreis]

It was actually saying that there wasn't enough space to link correctly, not that the link was badly defined.

@alees24 Have increased the total SRAM to 128 kB, When it gets merged we won't have this issue for a while.

[marnovandermaas]

This looks reasonable to me. I have not tested it so it would be good to have someone with more Rust knowledge to approve it as well.