Set grub timeout style

Set the grub timeout style to display the menu. By default it set to
'hidden' but can be changed to 'menu' to display the menu and then
wait for the timeout expire before booting the default entry.

Change-Id: I8c58407ef645d528dd77efe866bfe0389cbbbd33
Signed-off-by: Maksim Malchuk <maksim.malchuk@gmail.com>

diskimage_builder/elements/bootloader/README.rst

@@ -10,6 +10,11 @@ Arguments
 * ``DIB_GRUB_TIMEOUT`` sets the ``grub`` menu timeout.  It defaults to
   5 seconds.  Set this to 0 (no timeout) for fast boot times.
 
+* ``DIB_GRUB_TIMEOUT_STYLE`` sets the visibility of the ``grub`` menu.
+  It defaults to ``hidden`` (or ``countdown`` as an alias). Set this to
+  ``menu`` to display the menu and then wait for the timeout set by
+  ``DIB_GRUB_TIMEOUT`` to expire before booting the default entry.
+
 * ``DIB_BOOTLOADER_DEFAULT_CMDLINE`` sets parameters that are appended
   to the ``GRUB_CMDLINE_LINUX_DEFAULT`` values in ``grub.cfg``
   configuration. It defaults to ``nofb nomodeset gfxpayload=text``.

diskimage_builder/elements/bootloader/finalise.d/50-bootloader

@@ -85,6 +85,7 @@ fi
 echo "GRUB_DEVICE=LABEL=${DIB_ROOT_LABEL}" >> /etc/default/grub
 echo 'GRUB_DISABLE_LINUX_UUID=true' >> /etc/default/grub
 echo "GRUB_TIMEOUT=${DIB_GRUB_TIMEOUT:-5}" >>/etc/default/grub
+echo "GRUB_TIMEOUT_STYLE=${DIB_GRUB_TIMEOUT_STYLE:-hidden}" >>/etc/default/grub
 echo 'GRUB_TERMINAL="serial console"' >>/etc/default/grub
 echo 'GRUB_GFXPAYLOAD_LINUX=auto' >>/etc/default/grub
 

releasenotes/notes/grub-timeout-style-4bb9b2bde4ff2b02.yaml

@@ -0,0 +1,7 @@
+---
+features:
+  - |
+    The ``bootloader`` element will explicitly set the timeout style to
+    ``hidden`` to hide the menu. Set ``DIB_GRUB_TIMEOUT_STYLE`` to ``menu``
+    to display the menu and then wait for the timeout expire before booting
+    the default entry.