cargo_util: pass RUSTFLAGS to cargo build

classes/cargo_util.bbclass

@@ -36,12 +36,14 @@ 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 ??= ""
 export 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.
 export 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} $@"