build: phase 2 wave A — services build as packages

init, fat, display, display-demo, device-manager, input, logger, and
the two discovery fillers (acpi, fdt — each exporting an artifact named
"discovery"; the root -Ddiscovery picks which ships) convert to binary
packages on the pci-bus template. init's serial heartbeat flag rides
the dependency options (the root forwards its -Dserial). fat's and
display's unit tests move into their packages and the root aggregate
delegates to them. Boot-image file list unchanged.

system/services/fdt/build.zig

@@ -0,0 +1,17 @@
+//! The fdt discovery service as a binary package (docs/build-packages-plan.md,
+//! phase 2). The artifact is named "discovery" like acpi's: the Raspberry Pis
+//! hand over a flattened device tree instead of ACPI, and the aarch64 target
+//! flips the root build's -Ddiscovery default when it lands (docs/arm.md).
+//! A placeholder until the ARM bring-up.
+
+const std = @import("std");
+const build_support = @import("build-support");
+
+pub fn build(b: *std.Build) void {
+    const exe = build_support.userBinary(b, .{
+        .name = "discovery",
+        .root_source_file = b.path("fdt.zig"),
+        .domains = build_support.domains(b),
+    });
+    b.installArtifact(exe);
+}