Merge #593

593: RTIC macro expansion: Try to find target-dir r=korken89 a=AfoHT

Seems over-engineered, but for projects where 

```
[build]
target-dir = "target"
```
is set to anything other than default `target` RTIC did simply not produce any `rtic-expansion.rs`.

This changes the approach to not giving up if not finding `target/` by looking at `OUT_DIR` and traversing back until `TARGET` is found.

As the `TARGET` target-triple variable is not available, approximate the `TARGET` folder (found in `target-dir`) with `s.starts_with("thumbv")`.

`target-dir` as set in `.cargo/config` will now be the parent directory of the `Path` ending with `TARGET`

## Example running with default target:
```
cortex-m-rtic on  expansionoutdir [$!?] is 📦 v1.0.0 via R v1.58.0 took 4s
❯ cargo build --example spawn --target thumbv7em-none-eabihf
OUT_DIR
"/home/henrik/rust/rtic/cortex-m-rtic/target/thumbv7em-none-eabihf/debug/build/cortex-m-rtic-5bd81e8412a790d5/out"

target/ exists

Write file:
target/rtic-expansion.rs

    Finished dev [unoptimized + debuginfo] target(s) in 7.20s

```

## Contrived example
With `.cargo/config` containing:

```
[build]
target-dir = "/tmp/cargothingy/../rust/./target/cargo"`
```

```
cortex-m-rtic on  expansionoutdir [$!?] is 📦 v1.0.0 via R v1.58.0 took 3s
❯ cargo build --example spawn --target thumbv7em-none-eabihf
OUT_DIR
"/tmp/cargothingy/../rust/./target/cargo/thumbv7em-none-eabihf/debug/build/cortex-m-rtic-5bd81e8412a790d5/out"
"/tmp/cargothingy/../rust/./target/cargo"

Write file:
/tmp/cargothingy/../rust/./target/cargo/rtic-expansion.rs

    Finished dev [unoptimized + debuginfo] target(s) in 6.42s

```

## Less extreme with relative paths
```
[build]
target-dir = "../../cargothingy/target/buildfiles/and-stuff"
```

```
OUT_DIR
"/home/henrik/rust/rtic/cortex-m-rtic/../../cargothingy/target/buildfiles/and-stuff/thumbv7em-none-eabihf/debug/build/cortex-m-rtic-5bd81e8412a790d5/out"
"/home/henrik/rust/rtic/cortex-m-rtic/../../cargothingy/target/buildfiles/and-stuff"

Write file:
/home/henrik/rust/rtic/cortex-m-rtic/../../cargothingy/target/buildfiles/and-stuff/rtic-expansion.rs

    Finished dev [unoptimized + debuginfo] target(s) in 6.78s

```

Note: If the user creates a folder named target in the same directory where `Cargo.toml`/crate root is, that will be used for storing the expansion.

```
<...>
OUT_DIR
"/home/henrik/rust/rtic/cortex-m-rtic/../../cargothingy/target/buildfiles/and-stuff/thumbv7em-none-eabihf/debug/build/cortex-m-rtic-5bd81e8412a790d5/out"

target/ exists

Write file:
target/rtic-expansion.rs

    Finished dev [unoptimized + debuginfo] target(s) in 6.62s
```


Co-authored-by: Henrik Tjäder <henrik@grepit.se>

0 files changed, 0 insertions, 0 deletions