[uc20] snapd needs a way to incorporate firmware update with fwupd

For now, the actual EFI vfat partition is covered by ubuntu-seed which is mounted at /run/mnt/ubuntu-seed, and /boot/efi is not used. This will result in uefi-fw-tools (confined fwupd) can’t update firmware due to failing to search for ESP path. The fwupd daemon will find these paths in
order:

/boot/efi
/boot/EFI
/efi

The initial idea is that snapd can do bind mount for /boot/efi -> /run/mnt/ubuntu-seed. Once /boot/efi is mounted in the host, the profile “snap-update-ns.uefi-fw-tools” should get the benefit of doing rbind to /boot from /var/lib/snapd/hostfs/boot in the snap namespace.

However, the critical issue after that is that updating firmware also impacts TPM measurement, so that snapd/initramfs may fail to unseal the encryption, unless the system enters recovery mode to reseal the encrypted disk. We need to have more discussions about how snapd deals with the case of PCRs changing.