Set cargo cwd to manifest dir, workaround for cargo issue 10098

Cargo documentation for configuration toml says that a .cargo/config
checked into revision control in the project root should be picked up by
cargo, but in fact cargo only searches relative to cwd.  In order to
achieve the documented result, cargo needs to be invoked with the
current working directory already set to the directory containing the
manifest file.

see rust-lang/cargo#10098

classes/cargo.bbclass

@@ -43,7 +43,10 @@ oe_cargo_build () {
 	bbnote "cargo = $(which ${CARGO})"
 	bbnote "rustc = $(which ${RUSTC})"
 	bbnote "${CARGO} build ${CARGO_BUILD_FLAGS} $@"
-	"${CARGO}" build ${CARGO_BUILD_FLAGS} "$@"
+	# This is needed to pick up .cargo/config.toml in the project git root
+	# see https://github.com/rust-lang/cargo/issues/10098
+	CARGO_WORKINGDIR="$(dirname "${MANIFEST_PATH}")"
+	(cd "${CARGO_WORKINGDIR}"; "${CARGO}" build ${CARGO_BUILD_FLAGS} "$@")
 }
 
 do_compile[progress] = "outof:\s+(\d+)/(\d+)"