Switch Metropolis to EROFS-based root filesystem

This gets rid of the old large initramfs and switches to an EROFS-based root
filesystem. It also drops the copy & remount compatibility code. As this filesystem is
properly read-only and not just ephemeral, this also brings various changes to the code
to make systems compatible with that.

Test Plan: Covered by E2E tests, also manually smoke-tested.

X-Origin-Diff: phab/D696
GitOrigin-RevId: 037f2b8253e7cff8435cc79771fad05f53670ff0
diff --git a/metropolis/node/core/BUILD.bazel b/metropolis/node/core/BUILD.bazel
index 95344e8..28f4d2a 100644
--- a/metropolis/node/core/BUILD.bazel
+++ b/metropolis/node/core/BUILD.bazel
@@ -6,7 +6,7 @@
     srcs = [
         "debug_service.go",
         "main.go",
-        "switchroot.go",
+        "mounts.go",
     ] + select({
         "//metropolis/node:debug_build": ["delve_enabled.go"],
         "//conditions:default": ["delve_disabled.go"],