cargo_common.bbclass: improve operation with devtool

Because devtool inhibits all the normal bitbake fetchers, we can't mask crates.io.
Use EXTERNALSRC to detect if we're being built with devtool, and only mask
crates.io if we're not.

Fixes #195

classes/cargo_common.bbclass

@@ -21,7 +21,7 @@ export CARGO_HOME = "${WORKDIR}/cargo_home"
 export PKG_CONFIG_ALLOW_CROSS = "1"
 
 cargo_common_do_configure () {
-	mkdir -p ${CARGO_HOME}
+	mkdir -p ${CARGO_HOME}/bitbake
 	echo "paths = [" > ${CARGO_HOME}/config
 
 	for p in ${EXTRA_OECARGO_PATHS}; do
@@ -33,12 +33,16 @@ cargo_common_do_configure () {
 	cat <<- EOF >> ${CARGO_HOME}/config
 	[source.bitbake]
 	directory = "${CARGO_HOME}/bitbake"
-
-	[source.crates-io]
-	replace-with = "bitbake"
-	local-registry = "/nonexistant"
 	EOF
 
+	if [ "${EXTERNALSRC}" == "" ]; then
+		cat <<- EOF >> ${CARGO_HOME}/config
+		[source.crates-io]
+		replace-with = "bitbake"
+		local-registry = "/nonexistant"
+		EOF
+	fi
+
 	echo "[target.${HOST_SYS}]" >> ${CARGO_HOME}/config
 	echo "linker = '${RUST_TARGET_CCLD}'" >> ${CARGO_HOME}/config
 	if [ "${HOST_SYS}" != "${BUILD_SYS}" ]; then