modules: rename packages/kernel_modules to packages/lkmc

Then inside, split packages/lkmc into kernel_modules and userland,
to keep userland out of the kernel_modules parent path, which makes no
sense.

Copy built modules and userland to the output rootfs overlay.

Document Linux distro tradeoffs.

build-userland

@@ -61,5 +61,10 @@ else:
         cwd=common.userland_src_dir,
         extra_paths=[common.ccache_dir],
     ) == 0
+    common.copy_dir_if_update_non_recursive(
+        srcdir=common.userland_build_dir,
+        destdir=common.out_rootfs_overlay_dir,
+        filter_ext=common.executable_ext,
+    )
     end_time = time.time()
     common.print_time(end_time - start_time)