init: Add Ubuntu support for module loading and entropy initialization

[jiechen0826]

Fix UVM boot failures on Ubuntu-based rootfs where critical kernel modules
(hv_netvsc, hv_sock, bridge, etc.) are loadable (=m) rather than built-in (=y).

Problem

On Ubuntu, unlike Mariner, networking and vsock modules are not compiled into the kernel.
This causes two boot failures:

1. NIC hot-add timeout: When the host attaches a synthetic NIC to the UVM, no driver
   claims the vmbus device because hv_netvsc isn't loaded, resulting in:

timed out waiting for /sys/bus/vmbus/devices/<guid>/net to exist

2. Entropy init failure: init_entropy() was called before load_all_modules(),
   so hv_sock (vsock transport) wasn't available yet.

Changes

Commit 1: Move entropy init after module loading

• Move init_entropy() after load_all_modules() so that hv_sock is available
  for vsock-based entropy seeding.

Commit 2: Preload networking modules + logging improvements

• Add preload_networking_modules() to load critical modules by name
  (kmod_module_new_from_name) before the general ftw()-based module scan.
  The general scan walks by file path and may not reliably resolve all modules
  on Ubuntu's directory layout.
• Enable KMOD=1 in the default Makefile.
• Improve module load logging in load_module() and parse_tree_entry() with
  structured dmesg messages including error codes.

Impact on AzLinux based kernel

No impact. On AzLinux where these modules are built-in (=y):

• preload_networking_modules() calls kmod_module_probe_insert_module() which
  returns immediately (KMOD_PROBE_IGNORE_LOADED).
• The entropy reorder is safe — init_entropy() still runs after modules, just
  in a different position in main().