Pass --bin argument when building bootloader

This allows the bootloader to use the `required-features` configuration. Without the `--bin` argument cargo just omits all binaries with unsatisfied required features, so that an unrelated 'no bootloader executable found' error would occur.

Author: phil-opp

src/builder.rs

@@ -225,6 +225,7 @@ impl Builder {
             cmd.arg("xbuild");
             cmd.arg("--manifest-path");
             cmd.arg(&bootloader_pkg.manifest_path);
+            cmd.arg("--bin").arg(&bootloader_name);
             cmd.arg("--target-dir").arg(&target_dir);
             cmd.arg("--features")
                 .arg(bootloader_features.as_slice().join(" "));