Update build process to integrate with kernel patches

hello-world/Kbuild

@@ -1,10 +1,13 @@
 obj-m := helloworld.o
 helloworld-objs := hello_world.rust.o
 
+# Don't define these if the kernel we're compiling against already has them.
+ifndef CONFIG_RUST
 CARGO ?= cargo
 
 $(src)/target/x86_64-linux-kernel/debug/libhello_world.a: $(src)/Cargo.toml $(wildcard $(src)/src/*.rs)
 	cd $(src); env -u MAKE -u MAKEFLAGS $(CARGO) build -Z build-std=core,alloc --target=x86_64-linux-kernel
 
 %.rust.o: target/x86_64-linux-kernel/debug/lib%.a
 	$(LD) -r -o $@ --whole-archive $<
+endif

tests/Kbuild

@@ -1,10 +1,13 @@
 obj-m := testmodule.o
 testmodule-objs := $(TEST_NAME).rust.o
 
+# Don't define these if the kernel we're compiling against already has them.
+ifndef CONFIG_RUST
 CARGO ?= cargo
 
 $(src)/target/x86_64-linux-kernel/debug/lib%.a: $(src)/$(TEST_PATH)/Cargo.toml $(wildcard $(src)/$(TEST_PATH)/src/*.rs)
 	cd $(src)/$(TEST_PATH); env -u MAKE -u MAKEFLAGS CARGO_TARGET_DIR=../target $(CARGO) build -Z build-std=core,alloc --target=x86_64-linux-kernel
 
 %.rust.o: target/x86_64-linux-kernel/debug/lib%.a
 	$(LD) -r -o $@ --whole-archive $<
+endif