Allow of way to toggle freestanding platform features (#356)

* Allow of way to toggle freestanding platform features

`RUST_MBED_C_COMPILER_FREESTANDING=1` allows to skip cmake checks where
the target must produce valid binaries

* Rename the env variable and don't affect features

mbedtls-sys/build/cmake.rs

@@ -28,11 +28,16 @@ impl super::BuildConfig {
             cmk.define("CMAKE_C_COMPILER_FORCED", "TRUE");
         }
 
+        println!("cargo:rerun-if-env-changed=RUST_MBED_C_COMPILER_BAREMETAL");
+        let c_compiler_baremetal = std::env::var("RUST_MBED_C_COMPILER_BAREMETAL")
+            .map(|val| val == "1")
+            .unwrap_or_default();
+
         let target = std::env::var("TARGET").expect("TARGET environment variable should be set in build scripts");
         // thumbv6m-none-eabi, thumbv7em-none-eabi, thumbv7em-none-eabihf,
         // thumbv7m-none-eabi probably use arm-none-eabi-gcc which can cause the
         // cmake compiler test to fail.
-        if target.starts_with("thumbv") && target.contains("none-eabi") {
+        if target.starts_with("thumbv") && target.contains("none-eabi") || c_compiler_baremetal {
             // When building on Linux, -rdynamic flag is added automatically. Changing the
             // CMAKE_SYSTEM_NAME to Generic avoids this.
             cmk.define("CMAKE_SYSTEM_NAME", "Generic");