Build core with separate initramfs

Build the initramfs separately and include it via mkimage. Also includes
a patch to the kernel which adds support for hardcoded cmdline
to the Linux efistub.

This lowers build times by a lot, for normal changes they are now
below 5s

Test Plan: Ran `bazel run //core/scripts:launch`

X-Origin-Diff: phab/D245
GitOrigin-RevId: 206c7c5c979c10ffd25c36dfefd8b9290a6a3f43
diff --git a/core/BUILD b/core/BUILD
index d9ed017..edced0a 100644
--- a/core/BUILD
+++ b/core/BUILD
@@ -1,15 +1,18 @@
 genrule(
     name = "image",
     srcs = [
-        "@//core/cmd/mkimage",
-        "@//core/build/linux_kernel:image",
+        "@//core/build/linux_kernel:bzImage",
+        "@//core/build/linux_kernel:initramfs",
     ],
     outs = [
         "smalltown.img",
     ],
     cmd = """
-    $(location @//core/cmd/mkimage) $(location @//core/build/linux_kernel:image) $@
+    $(location @//core/cmd/mkimage) $(location @//core/build/linux_kernel:bzImage) $@ $(location @//core/build/linux_kernel:initramfs)
     """,
+    tools = [
+        "@//core/cmd/mkimage",
+    ],
     visibility = ["//visibility:public"],
 )