Fix build.rs in project readme

[StefanB7]

This morning I've been experimenting with FreeRTOS-rust on an STM32F411RE Nucleo board. When setting up my project, I ran into some issues building the project when using the build.rs example provided in the readme. This PR fixes the issues I came across.

The errors I got:

error[E0308]: mismatched types                                                                                                                                                                                              
   --> build.rs:7:21
    |
7   |     b.freertos_port("GCC/ARM_CM3"); // Port dir relativ to 'FreeRTOS-Kernel/portable' 
    |       ------------- ^^^^^^^^^^^^^- help: try using a conversion method: `.to_string()`
    |       |             |
    |       |             expected `String`, found `&str`
    |       arguments to this method are incorrect
    |

was fixed by adding .to_owned().

   --> build.rs:8:12
    |
8   |     b.heap("heap4.c");              // Set the heap_?.c allocator to use from
    |       ---- ^^^^^^^^^- help: try using a conversion method: `.to_string()`
    |       |    |
    |       |    expected `String`, found `&str`
    |       arguments to this method are incorrect
    |

was fixed by creating a String from the string literal. After doing this, I got:

error[E0282]: type annotations needed
 --> build.rs:8:7
  |
8 |     b.heap(String::from("heap4.c"));              // Set the heap_?.c allocator to use from
  |       ^^^^ cannot infer type of the type parameter `P` declared on the associated function `heap`
  |
help: consider specifying the generic argument
  |
8 |     b.heap::<P>(String::from("heap4.c"));              // Set the heap_?.c allocator to use from
  |           +++++

which was solved by b.heap::<String>(......

error: format argument must be a string literal
  --> build.rs:13:45
   |
13 |     b.compile().unwrap_or_else(|e| { panic!(e.to_string()) });
   |                                             ^^^^^^^^^^^^^
   |
help: you might be missing a string literal to format with
   |
13 |     b.compile().unwrap_or_else(|e| { panic!("{}", e.to_string()) });
   |                                             +++++

I'll try to create a PR for an example of FreeRTOS-rust on the STM32F411 Nucleo board later today.
Thanks everyone for this project.

[schteve]

Thanks for this! The readme example should definitely be correct and easy to drop into a new project.

Some of what you're seeing may be already fixed in the latest (unreleased) trunk. I think the string conversions and the turbofish on heap() should be unnecessary since we changed these functions to take an AsRef<Path>. So I'd rather not change those lines. However the change to the panic does make sense I think.

You should be able to confirm that the latest code works by changing your Cargo.toml to point to a local clone of the repo (freertos-cargo-build = { path = "../FreeRTOS-rust/freertos-cargo-build"} or similar).

We really need to make a new release at some point to get the latest changes out there and avoid issues like this @niondir