classes/cargo_common: ensure B is clean

The cargo class defaults to out-of-tree builds in WORKDIR/build, but
at no point was that directory cleaned.  This causes problems with the
rust standard library recipe (libstd-rs) which installs manually with cp,
so rebuilds can be contaminated with the contents of previous builds.

I believe that post-release we should switch cargo.bbclass to mandating
out-of-tree builds to reduce the complexity, but for now in out-of-tree
builds we can just delete the ${B}/target directory.

Note that we use ${B}/target because there at least were reasons to use
that name[1], it is unclear if these limitations still hold. We can't
simply clean ${B} because that will break recipes that use cargo and
something else to build, for example librsvg.

[1] rust-lang/cargo#1657

Signed-off-by: Ross Burton <[email protected]>
Signed-off-by: Richard Purdie <[email protected]>

Author: rossburton

meta/classes-recipe/cargo_common.bbclass

@@ -110,6 +110,10 @@ cargo_common_do_configure () {
 	# Put build output in build directory preferred by bitbake instead of
 	# inside source directory unless they are the same
 	if [ "${B}" != "${S}" ]; then
+		# We should consider mandating out-of-tree builds and just using [cleandirs]
+		rm -rf ${B}/target
+		mkdir -p ${B}
+
 		cat <<- EOF >> ${CARGO_HOME}/config.toml
 
 		[build]