classes/cargo: revert to utilizing compiler/linker wrappers

Don't supply the linker arguments via rustflags and then mangle with the
RUSTFLAGS environment variable. This is also a parial reversion of
4e1bda643e and 5afc0cc471. These changes are reverted since it assumed
people would build with cargo and not directly with rustc. ref #116.

classes/cargo.bbclass

@@ -46,29 +46,18 @@ replace-with = "local"
 registry = "https://github.com/rust-lang/crates.io-index"
 EOF
 
-	# We need to use the real Yocto linker and get the linker
-	# flags to it. Yocto has the concept of BUILD and TARGET
-	# and uses HOST to be the currently selected one. However
-	# LDFLAGS and TOOLCHAIN_OPTIONS are not prefixed with HOST
-	echo "[build]" >> ${CARGO_HOME}/config
-	echo "rustflags = [" >> ${CARGO_HOME}/config
-	echo "'-C'," >> ${CARGO_HOME}/config
-	echo "'link-args=${LDFLAGS}${HOST_CC_ARCH}${TOOLCHAIN_OPTIONS}'," >> ${CARGO_HOME}/config
-	for p in ${RUSTFLAGS}; do
-		printf "'%s'\n" "$p"
-	done | sed -e 's/$/,/' >> ${CARGO_HOME}/config
-	echo "]" >> ${CARGO_HOME}/config
 	echo "[target.${HOST_SYS}]" >> ${CARGO_HOME}/config
 	echo "linker = '${RUST_TARGET_CCLD}'" >> ${CARGO_HOME}/config
 }
 
+RUSTFLAGS ??= ""
 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.
 CARGO_TARGET_SUBDIR="${HOST_SYS}/release"
 oe_cargo_build () {
-	unset RUSTFLAGS
+	export RUSTFLAGS="${RUSTFLAGS}"
 	bbnote "cargo = $(which cargo)"
 	bbnote "rustc = $(which rustc)"
 	bbnote "${CARGO} build ${CARGO_BUILD_FLAGS} $@"