Provide an ability to disable serial console injection

By default, we attempt to inject a serial console, which may, or may not
be needed, for example, Centos Stream 9 cloud images already configure
a console setting, and repeating it just might be undesirable and cause
workload performance degredation if the kernel or an application has to
report anything to a console.

This change generally results in original console entries being preserved,
which might actually be a bug and get fixed in a latter patch.

Generally, users of dib *should* likely be specific what they want to do
with their console setting, and without setting the new
DIB_BOOTLOADER_USE_SERIAL_CONSOLE paramter to false, the default will
be adhered to, and any pre-existing serial console entries will *not* be
de-duplicated from the base image. That too is *likely* a bug, but a
harder one to fix.

Change-Id: Icdfb5ed021b1a91e2de3c9a22bb2ff7fe9882bcd

diskimage_builder/elements/bootloader/README.rst

@@ -19,6 +19,9 @@ Arguments
   to the ``GRUB_CMDLINE_LINUX_DEFAULT`` values in ``grub.cfg``
   configuration. It defaults to ``nofb nomodeset gfxpayload=text``.
 
+* ``DIB_BOOTLOADER_USE_SERIAL_CONSOLE`` allows usage of a serial console
+  to be disabled in the resulting image by setting to a value of ``False``.
+
 * ``DIB_BOOTLOADER_SERIAL_CONSOLE`` sets the serial device to be
   used as a console. It defaults to ``hvc0`` for PowerPC, 
   ``ttyAMA0,115200`` for ARM64, otherwise ``ttyS0,115200``.

diskimage_builder/elements/bootloader/environment.d/10-bootloader-default-cmdline

@@ -8,3 +8,4 @@ else
     export DIB_BOOTLOADER_VIRTUAL_TERMINAL=${DIB_BOOTLOADER_VIRTUAL_TERMINAL}
 fi
 export DIB_NO_TIMER_CHECK=${DIB_NO_TIMER_CHECK:-"True"}
+export DIB_BOOTLOADER_USE_SERIAL_CONSOLE=${DIB_BOOTLOADER_USE_SERIAL_CONSOLE:-"True"}

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

@@ -91,7 +91,11 @@ 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=${DIB_GRUB_TIMEOUT_STYLE:-hidden}" >>/etc/default/grub
-echo 'GRUB_TERMINAL="serial console"' >>/etc/default/grub
+if [[ "True" == "${DIB_BOOTLOADER_USE_SERIAL_CONSOLE:-True}" ]]; then
+    echo 'GRUB_TERMINAL="serial console"' >>/etc/default/grub
+else
+    echo 'GRUB_TERMINAL="console"' >>/etc/default/grub
+fi
 echo 'GRUB_GFXPAYLOAD_LINUX=auto' >>/etc/default/grub
 
 # NOTE(TheJulia): We need to remove any boot entry from the /etc/default/grub
@@ -139,16 +143,19 @@ if [[ -x /bin/fips-mode-setup ]]; then
 fi
 
 
-
-if [[ -n "${DIB_BOOTLOADER_SERIAL_CONSOLE}" ]]; then
+if [[ "True" == "${DIB_BOOTLOADER_USE_SERIAL_CONSOLE:-True}" ]]; then
+    if [[ -n "${DIB_BOOTLOADER_SERIAL_CONSOLE}" ]]; then
         SERIAL_CONSOLE="console=${DIB_BOOTLOADER_SERIAL_CONSOLE}"
-elif [[ "powerpc ppc64 ppc64le" =~ "$ARCH" ]]; then
+    elif [[ "powerpc ppc64 ppc64le" =~ "$ARCH" ]]; then
         # Serial console on Power is hvc0
         SERIAL_CONSOLE="console=hvc0"
-elif [[ "arm64" =~ "$ARCH" ]]; then
+    elif [[ "arm64" =~ "$ARCH" ]]; then
         SERIAL_CONSOLE="console=ttyAMA0,115200"
-else
+    else
         SERIAL_CONSOLE="console=ttyS0,115200"
+    fi
+else
+    SERIAL_CONSOLE=""
 fi
 
 if [[ -n "${DIB_BOOTLOADER_VIRTUAL_TERMINAL}" ]]; then
@@ -165,7 +172,11 @@ fi
 
 GRUB_CMDLINE_LINUX_DEFAULT="${VIRTUAL_TERMINAL} ${SERIAL_CONSOLE} ${NO_TIMER_CHECK}"
 echo "GRUB_CMDLINE_LINUX_DEFAULT=\"${GRUB_CMDLINE_LINUX_DEFAULT} ${DIB_BOOTLOADER_DEFAULT_CMDLINE}${BOOT_FS}${BOOT_FIPS}\"" >>/etc/default/grub
-echo 'GRUB_SERIAL_COMMAND="serial --speed=115200 --unit=0 --word=8 --parity=no --stop=1"' >>/etc/default/grub
+
+
+if [[ "True" == "${DIB_BOOTLOADER_USE_SERIAL_CONSOLE:-True}" ]]; then
+    echo 'GRUB_SERIAL_COMMAND="serial --speed=115200 --unit=0 --word=8 --parity=no --stop=1"' >>/etc/default/grub
+fi
 
 # os-prober leaks /dev/sda into config file in dual-boot host
 # Disable grub-os-prober to avoid the issue  while running

releasenotes/notes/bootloader-disable-serial-console-d6aaa42ef3031d86.yaml

@@ -0,0 +1,6 @@
+---
+features:
+  - |
+    Adds the ability to disable serial console logging from being setup in
+    the ``bootloader`` element by setting the
+    ``DIB_BOOTLOADER_USE_SERIAL_CONSOLE`` environment variable to ``False``.