hashes/embedded: Add script dir and README

To help folk work out how to run the `hashes/embedded` test crate copy
over the `script` directory and an updated version of the `README` from
`embedded/bitcoin`.

hashes/embedded/README.md

@@ -0,0 +1,30 @@
+# Running
+
+To run the embedded test, first prepare your environment:
+
+```shell
+sudo ./scripts/install-deps
+rustup +nightly target add thumbv7m-none-eabi
+```
+
+Then:
+
+```shell
+source ./scripts/env.sh && cargo +nightly run --target thumbv7m-none-eabi
+```
+
+Output should be something like:
+
+```text
+hash:ba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad hash_check:ba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad
+hash:ba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad hash_check:ba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad
+```
+
+## Cleanup
+
+After sourcing `scripts/env.sh` and _before_ building again using another target
+you'll want to unset `RUSTFLAGS` otherwise you'll get linker errors.
+
+```shell
+unset RUSTFLAGS
+```

hashes/embedded/scripts/env.sh

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+export RUSTFLAGS="-C link-arg=-Tlink.x"
+export CARGO_TARGET_THUMBV7M_NONE_EABI_RUNNER="qemu-system-arm -cpu cortex-m3 -machine mps2-an385 -nographic -semihosting-config enable=on,target=native -kernel"

hashes/embedded/scripts/install-deps

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+#!/bin/sh
+
+apt install gcc-arm-none-eabi qemu-system-arm gdb-multiarch