package/rust: set musl-root for musl toolchains

Without musl-root in config.toml the build stop with the following error:

thread 'main' panicked at 'couldn't find libc.a in musl libdir: output/host/arm-buildroot-linux-musleabihf/sysroot/lib/lib', src/bootstrap/sanity.rs:192:25

musl-root is needed to provide the path to libc.a and libunwind.a because
rust link them statically into the standard library and liblibc [1].

For other libc, musl-root is not used and ignored [2].

[1] cd980b3bee
[2] https://github.com/rust-lang/rust/blob/1.58.1/src/bootstrap/sanity.rs#L181

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
(cherry picked from commit a41b0f31de)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/rust/rust.mk

@@ -56,6 +56,7 @@ define HOST_RUST_CONFIGURE_CMDS
 		echo 'sysconfdir = "$(HOST_DIR)/etc"'; \
 		echo '[rust]'; \
 		echo 'channel = "stable"'; \
+		echo 'musl-root = "$(STAGING_DIR)"' ; \
 		echo '[target.$(RUSTC_TARGET_NAME)]'; \
 		echo 'cc = "$(TARGET_CROSS)gcc"'; \
 		echo '[llvm]'; \