treewide: move build helper to more fitting places

Change-Id: I3d0cfe9283222d403ae369ec9db09201ad511e15
Reviewed-on: https://review.monogon.dev/c/monogon/+/3327
Reviewed-by: Serge Bazanski <serge@monogon.tech>
Tested-by: Jenkins CI
diff --git a/osbase/build/mkcpio/BUILD.bazel b/osbase/build/mkcpio/BUILD.bazel
new file mode 100644
index 0000000..d281f6d
--- /dev/null
+++ b/osbase/build/mkcpio/BUILD.bazel
@@ -0,0 +1,20 @@
+load("@io_bazel_rules_go//go:def.bzl", "go_binary", "go_library")
+
+go_library(
+    name = "mkcpio_lib",
+    srcs = ["main.go"],
+    importpath = "source.monogon.dev/osbase/build/mkcpio",
+    visibility = ["//visibility:private"],
+    deps = [
+        "//osbase/build/fsspec",
+        "@com_github_cavaliergopher_cpio//:cpio",
+        "@com_github_klauspost_compress//zstd",
+        "@org_golang_x_sys//unix",
+    ],
+)
+
+go_binary(
+    name = "mkcpio",
+    embed = [":mkcpio_lib"],
+    visibility = ["//visibility:public"],
+)