Support for observer-a (#2493)

This creates a draft `observer` app in preparation for bringup. Issue:
#2409

---------

Co-authored-by: Matt Keeter <matt@oxide.computer>

Author: evan-oxide

.github/buildomat/jobs/observer-a.sh

@@ -0,0 +1,16 @@
+#!/bin/bash
+#:
+#: name = "build observer-a"
+#: variety = "basic"
+#: target = "ubuntu-22.04"
+#: rust_toolchain = true
+#: output_rules = [
+#:  "=/work/*.zip",
+#:  "=/work/this_is_not_signed.txt",
+#: ]
+
+set -o errexit
+set -o pipefail
+set -o xtrace
+
+exec .github/buildomat/build-one.sh observer-a app/observer/rev-a.toml default

Cargo.lock

@@ -0,0 +1,31 @@
+[package]
+edition = "2024"
+readme = "README.md"
+name = "observer"
+version = "0.1.0"
+
+[features]
+dump = ["kern/dump"]
+measurement-handoff = ["drv-stm32h7-startup/measurement-handoff"]
+
+[dependencies]
+cfg-if = { workspace = true }
+cortex-m = { workspace = true }
+cortex-m-rt = { workspace = true }
+stm32h7 = { workspace = true, features = ["rt", "stm32h753"] }
+
+drv-stm32h7-startup = { path = "../../drv/stm32h7-startup", features = ["h753"] }
+kern = { path = "../../sys/kern" }
+
+[build-dependencies]
+build-util = {path = "../../build/util"}
+
+# this lets you use `cargo fix`!
+[[bin]]
+name = "observer"
+test = false
+doctest = false
+bench = false
+
+[lints]
+workspace = true

app/observer/Cargo.toml

@@ -0,0 +1,9 @@
+# Observer Service Processor (SP) firmware
+
+The Observer interfaces with the power shelf in the Oxide rack.
+This hardware is the successor to the Power Shelf Controller (PSC).
+
+This folder contains the firmware that runs on its service processor (SP).
+
+The Root of Trust firmware is common across multiple boards and can be found
+in the [`oxide-rot-1` subfolder](../oxide-rot-1).