remove usages of rustup

The expectation is now that we will use the Rust compiler provided by
the hps-sdk package in Chrome OS. Remove the "+${RUST_VERSION}"
arguments which are rustup-specific and don't work with plain cargo.

Also remove the stuff in scripts/setup which checks for and installs
rustup, because we don't need it.

Add a rust-toolchain.toml file which matches the rustup configuration we
were previously doing in scripts/setup. This allows developers who
already have rustup on gLinux to continue running things outside the
chroot as before, to smooth the transition.

BUG=b:232880023
TEST=(chroot) scripts/run-tests  # it passes
TEST=(glinux) scripts/run-tests  # it passes

Change-Id: I41e639fa1ec8a425d6ed071ca892a7e8be9d9365
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/hps-firmware/+/3670600
Commit-Queue: Dan Callaghan <dcallagh@chromium.org>
Tested-by: Dan Callaghan <dcallagh@chromium.org>
Reviewed-by: David Lattimore <dml@chromium.org>
diff --git a/scripts/check-binary-sizes b/scripts/check-binary-sizes
index cfbcbf2..643b871 100755
--- a/scripts/check-binary-sizes
+++ b/scripts/check-binary-sizes
@@ -16,7 +16,7 @@
 APPLICATION="${PROJECT_ROOT}/rust/mcu/target/thumbv6m-none-eabi/release/stage1_app.bin"
 (
     cd "${PROJECT_ROOT}/rust/mcu"
-    cargo "+${RUST_VERSION}" build -p stage0 --release
+    cargo build -p stage0 --release
     arm-none-eabi-objcopy \
         -O binary \
         target/thumbv6m-none-eabi/release/stage0 \
@@ -24,7 +24,7 @@
 )
 (
     cd "${PROJECT_ROOT}/rust/mcu"
-    cargo "+${RUST_VERSION}" build -p stage1_app --release
+    cargo build -p stage1_app --release
     arm-none-eabi-objcopy \
         -O binary \
         target/thumbv6m-none-eabi/release/stage1_app \