MAAS can specify kernel boot options to machines on both a global basis (UI and CLI) and a per-machine basis (CLI-only). A full catalogue of available options can be found in the Linux kernel parameters list at kernel.org.
To set kernel boot options globally, as an admin, open the ‘Settings’ page and on the ‘General’ tab scroll down to the ‘Global Kernel Parameters’ section:
Type in the desired (space separated) options and click ‘Save’. The contents of the field will be used as-is. Do not use extra characters.