Verify a test boot for Morello uses the right kernel.

When booting an image with a non-default kernel, expect needs to
interrupt the loader and select the alternate kernel before booting.
If the boot gets to single user (mounting root) without interrupt the
loader, fail explicitly.

pycheribuild/boot_cheribsd/__init__.py

@@ -845,6 +845,7 @@ def boot_and_login(child: CheriBSDSpawnMixin, *, starttime, kernel_init_only=Fal
         loader_boot_prompt_messages = [*boot_messages, BOOT_LOADER_PROMPT]
         loader_boot_messages = [*loader_boot_prompt_messages, AUTOBOOT_PROMPT]
         i = child.expect(loader_boot_messages, timeout=20 * 60, timeout_msg="timeout before loader or kernel")
+        ran_manual_boot = False
         if i >= len(boot_messages):
             # Skip 10s wait from loader(8) if we see the autoboot message
             if i == loader_boot_messages.index(AUTOBOOT_PROMPT):  # Hit Enter
@@ -855,15 +856,18 @@ def boot_and_login(child: CheriBSDSpawnMixin, *, starttime, kernel_init_only=Fal
                     i = child.expect(loader_boot_prompt_messages, timeout=60,
                                      timeout_msg="timeout before loader prompt")
                     if i != loader_boot_prompt_messages.index(BOOT_LOADER_PROMPT):
-                        failure("failed to enter boot loader prompt", exit=True)
+                        failure("failed to enter boot loader prompt after stopping autoboot", exit=True)
                         # Fall through to BOOT_LOADER_PROMPT
                 else:
                     child.sendline("\r")
             if i == loader_boot_messages.index(BOOT_LOADER_PROMPT):  # loader(8) prompt
                 success("===> loader(8) waiting boot commands")
                 # Just boot the default kernel if no alternate kernel directory is given
                 child.sendline("boot {}".format(boot_alternate_kernel_dir or ""))
+                ran_manual_boot = True
             i = child.expect(boot_messages, timeout=5 * 60, timeout_msg="timeout before kernel")
+        if boot_alternate_kernel_dir and not ran_manual_boot:
+            failure("failed to enter boot loader prompt", exit=True)
         if i == boot_messages.index(TRYING_TO_MOUNT_ROOT):
             success("===> mounting rootfs")
             if bootverbose: