T7397: add "system kernel option quiet" to suppress boot messages

docs/configuration/system/option.rst

@@ -56,22 +56,27 @@
 
     The available modes are:
 
     * ``active`` This is the low-level firmware control mode based on the profile
       set and the system governor has no effect.
     * ``passive`` The driver allows the system governor to manage CPU frequency
       while providing available performance states.
     * ``guided`` The driver allows to set desired performance levels and the firmware
       selects a performance level in this range and fitting to the current workload.
 
     This will add the following two options to the Kernel commandline:
 
     * ``initcall_blacklist=acpi_cpufreq_init`` Disable default ACPI CPU frequency scale
     * ``amd_pstate={mode}`` Sets the p-state mode
 
     .. note:: Setting will only become active with the next reboot!
 
     .. seealso:: https://docs.kernel.org/admin-guide/pm/amd-pstate.html
 
+.. cfgcmd:: set system option kernel quiet
+
+    Suppress most kernel messages during boot. This is useful for systems with
+    embedded serial console interfaces to speed up the boot process.
+
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@@ -108,7 +113,7 @@
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 When starting a VyOS live system (the installation CD) the configured keyboard
 layout defaults to US. As this might not suite everyone's use case you can adjust
 the used keyboard layout on the system console.
 
 .. cfgcmd:: set system option keyboard-layout <us | fr | de | fi | no | dk>