m/node: build microcode payloads

This adds a builder for loadable microcode payloads for the Linux
kernel and microcode for Intel and AMD CPUs. It also adds a rule
generating a microcode payload for Metropolis at
//metropolis/node:ucode but does not integrate it yet.

Change-Id: I00145e4c983d9ff3e81881e92cbecc3e09392665
Reviewed-on: https://review.monogon.dev/c/monogon/+/546
Reviewed-by: Sergiusz Bazanski <serge@monogon.tech>
diff --git a/WORKSPACE b/WORKSPACE
index f6b1155..c9464b9 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -144,6 +144,13 @@
     version = "20211216",
 )
 
+load("//third_party/intel_ucode:external.bzl", "intel_ucode_external")
+
+intel_ucode_external(
+    name = "intel_ucode",
+    version = "20220207",
+)
+
 load("//third_party/edk2:external.bzl", "edk2_external")
 
 edk2_external(name = "edk2")