Set grub timeout default

Set the grub timeout to 5 seconds by default, and add notes on how to
update this.  This will stop infra having to carry an element that
goes and rewrites the grub configuration.

Change-Id: I556b3f48eff1b67ee8c4b9b64f749af95100fb99

elements/bootloader/README.rst

@@ -1,7 +1,14 @@
 ==========
 bootloader
 ==========
-Installs grub[2] on boot partition on the system. In case GRUB2
-is not available in the system, a fallback to Extlinux will happen. It's
-also possible to enforce the use of Extlinux by exporting a DIB_EXTLINUX
-variable to the environment.
+
+Installs ``grub[2]`` on boot partition on the system. In case GRUB2 is
+not available in the system, a fallback to Extlinux will happen. It's
+also possible to enforce the use of Extlinux by exporting a
+``DIB_EXTLINUX`` variable to the environment.
+
+Arguments
+=========
+
+* ``DIB_GRUB_TIMEOUT`` sets the ``grub`` menu timeout.  It defaults to
+  5 seconds.  Set this to 0 (no timeout) for fast boot times.

elements/bootloader/finalise.d/50-bootloader

@@ -133,6 +133,7 @@ function install_grub2 {
         GRUB_CFG=/boot/grub/grub.cfg
     fi
 
+    echo "GRUB_TIMEOUT=${DIB_GRUB_TIMEOUT:-5}" >>/etc/default/grub
     echo 'GRUB_TERMINAL="serial console"' >>/etc/default/grub
     echo 'GRUB_GFXPAYLOAD_LINUX=text' >>/etc/default/grub
     echo 'GRUB_CMDLINE_LINUX_DEFAULT="console=tty0 console=ttyS0,115200 no_timer_check"' >>/etc/default/grub

releasenotes/notes/grub-timeout-1cdd14a2b1467d89.yaml

@@ -0,0 +1,6 @@
+---
+features:
+  - The ``bootloader`` element will explicitly set the timeout to
+    ``5`` seconds when using ``grub`` (previously this was undefined,
+    but platform defaults were usually 5 seconds).  Set this to ``0``
+    for faster boots.