Add --enable-earlycon option to vm tool

This should make it easier to debug issues that occur before
virtio_console is available.

The code is guarded by the new RELEASE_AVF_IMPROVE_DEBUGGABLE_VMS
trunk-stable flag. This patch also guards the corresponding change in
pvmfw with the same flag just for the sake of consistency.

This is effectively a relanding of
ec68eaf6e8ba07cf400cdc33501cdd1a85dfe665 with a minor fix to avoid build
breakage.

Test: vm run-microdroid --protected --enable-earlycon --debug full
Test: vm run-microdroid --protected --enable-earlycon --debug none
Test: atest MicrodroidTests
Change-Id: Ic01b35a633456d2fc202374c10da1e22a83ee23d
diff --git a/guest/pvmfw/src/entry.rs b/guest/pvmfw/src/entry.rs
index ce04317..8f9340b 100644
--- a/guest/pvmfw/src/entry.rs
+++ b/guest/pvmfw/src/entry.rs
@@ -276,8 +276,9 @@
     MEMORY.lock().as_mut().unwrap().unshare_all_memory();
 
     if let Some(mmio_guard) = get_mmio_guard() {
-        // Keep UART MMIO_GUARD-ed for debuggable payloads, to enable earlycon.
-        if !debuggable_payload {
+        if cfg!(debuggable_vms_improvements) && debuggable_payload {
+            // Keep UART MMIO_GUARD-ed for debuggable payloads, to enable earlycon.
+        } else {
             mmio_guard.unmap(UART_PAGE_ADDR).map_err(|e| {
                 error!("Failed to unshare the UART: {e}");
                 RebootReason::InternalError