While debugging https://bugzilla.opensuse.org/show_bug.cgi?id=1129466, I found
a problem with current fwupd, transactional update system (MicroOS) and UEFI
Capsule update.

To reproduce:
- edit /etc/fwupd/uefi_capsule.conf and uncomment in section [uefi_capsule]
EnableGrubChainLoad=true

- ensure you have an update requiring uefi capsule pending
- run fwupdmgr update
- check journal logs (journalctl -u fwupd.service):

fwupd[29391]: /usr/sbin/grub2-mkconfig: ligne 306: /boot/grub2/grub.cfg.new:
Read-only file-system
fwupd[29786]: /usr/bin/grub2-editenv���: error���: can't open ������/boot/grub2/grubenv
������: Read-only file-system


fwupdmgr should be modified to call transactional-update grub.cfg instead of
grub2-mkconfig on transactional system.