cargo.bbclass: supply linker and link-args via rustflags

Supply the linker and the linker args via rustflags which should make it
not necessary to specify them via target JSONs. We explicitly cannot use
the RUSTFLAGS environment variable do to the way cargo parses arguments
and passes them to rustc.

classes/cargo.bbclass

@@ -1,6 +1,7 @@
 inherit rust-vars
 # add crate fetch support
 inherit crate-fetch
+inherit rust-triples
 
 # the binary we will use
 CARGO = "cargo"
@@ -47,6 +48,15 @@ local-registry = "${WORKDIR}/cargo_registry"
 replace-with = "local"
 registry = "https://github.com/rust-lang/crates.io-index"
 EOF
+
+    # We need to use the real Yocto linker and get the linker
+    # flags to it. Yocto has the concept of BUILD and TARGET
+    # and uses HOST to be the currently selected one. However
+    # LDFLAGS and TOOLCHAIN_OPTIONS are not prefixed with HOST
+    echo "[build]" >> ${CARGO_HOME}/config
+    echo "rustflags = ['-C', 'link-args=${LDFLAGS}${HOST_CC_ARCH}${TOOLCHAIN_OPTIONS}']" >> ${CARGO_HOME}/config
+    echo "[target.${RUST_HOST_SYS}]" >> ${CARGO_HOME}/config
+    echo "linker = '${HOST_PREFIX}gcc'" >> ${CARGO_HOME}/config
 }
 
 # All the rust & cargo ecosystem assume that CC, LD, etc are a path to a single
@@ -59,14 +69,12 @@ export RUST_CFLAGS = "${HOST_CC_ARCH}${TOOLCHAIN_OPTIONS} ${CFLAGS}"
 export RUST_BUILD_CC = "${CCACHE}${BUILD_PREFIX}gcc"
 export RUST_BUILD_CFLAGS = "${BUILD_CC_ARCH} ${BUILD_CFLAGS}"
 
-RUSTFLAGS ??= ""
 CARGO_BUILD_FLAGS = "-v --target ${HOST_SYS} --release"
 
 # This is based on the content of CARGO_BUILD_FLAGS and generally will need to
 # change if CARGO_BUILD_FLAGS changes.
 CARGO_TARGET_SUBDIR="${HOST_SYS}/release"
 oe_cargo_build () {
-	export RUSTFLAGS="${RUSTFLAGS}"
 	bbnote "cargo = $(which cargo)"
 	bbnote "rustc = $(which rustc)"
 	bbnote "${CARGO} build ${CARGO_BUILD_FLAGS} $@"