Add CONFIG_BOOT_XEN_ADD CONFIG_BOOT_XEN_REMOVE

[tlaurion]

One of the interesting functionalities Heads now provides through config heads configs management is resetting/merging /etc/config.user(live) + /etc/config (stock).

This facilitates testing of different configurations on live system when unsafe booting since we invalidated measurements by going into recovery and modified PCR4, prohibiting release of the TPM Disk Unlock Key from TPM since measurements are invalidated.

For OS testings, where end user can modify /etc/config.user and then call config-gui.sh to inject that new config into cbfs, seal it on next reboot, only OS kernel options can be modified from compiled board config which is placed under /etc/config in rom:

Example of current ADD/REMOVE board config options usable right now:

export CONFIG_BOOT_KERNEL_ADD="intel_iommu=on intel_iommu=igfx_off"
export CONFIG_BOOT_KERNEL_REMOVE="quiet"

That permits to tinker and tweak OS bootable kernel options, where Xen config tweaks still need to be manually modified inside of grub config and grub config needs to be regenerated

I was thinking of adding CONFIG_BOOT_XEN_ADD and CONFIG_BOOT_XEN_REMOVE options in the set.

@MrChromebox ?

[tlaurion]

Might be needed for #1910 considering latest Xen speculation mitigations slow down the d16 to turtle speed, where sys-net and sys-usb would need to be switched to pv instead of hvm mode see https://15h.org/index.php/QubesOS

[tlaurion]

Note that it would be nice if Heads could permit to modify this through configuration settings option, so that the docs could guide the user to specify such overrides to be applied once for the current boot, or save it under config.user(persist settings and any other configuration settings changes), which is stored and measured from CBFS after flashing the changes to CBFS, making those overrides persistent across reboots, and requiring user to reseal TOTPM and TPM DUK since firmware measurements changed.

[tlaurion]

Again not so important considering that d16 needs manual switching from within qubesos for pci service-qubes (sys-net, sys-usb) alongside of passing xen additional arguments in grub as documented at https://15h.org/index.php/Qubes_OS

Input for #1929