Factor in CARGO_SRC_DIR when setting MANIFEST_PATH.

classes/cargo.bbclass

@@ -22,8 +22,12 @@ B = "${WORKDIR}/build"
 # where the issue occured
 export RUST_BACKTRACE = "1"
 
-# Assume there's a Cargo.toml directly in the source directory
-MANIFEST_PATH ??= "${S}/Cargo.toml"
+# The directory of the Cargo.toml relative to the root directory, per default
+# assume there's a Cargo.toml directly in the root directory
+CARGO_SRC_DIR ??= ""
+
+# The actual path to the Cargo.toml
+MANIFEST_PATH ??= "${S}/${CARGO_SRC_DIR}/Cargo.toml"
 
 RUSTFLAGS ??= ""
 BUILD_MODE = "${@['--release', ''][d.getVar('DEBUG_BUILD') == '1']}"