uefi: Check that EFI system partition is mounted (Fixes: #436)
uefi: Check that EFI system partition is mounted (Fixes: #436)

If the ESP isn't mounted it's possible that the update may look
succesful on the Linux side, but certain to fail later.

fwupd/fwupd,fwupd/fwupd,vathpela/fwupd,fwupd/fwupd,superm1/fwupd,vathpela/fwupd,vathpela/fwupd,hughsie/fwupd,hughsie/fwupd,hughsie/fwupd,fwupd/fwupd,superm1/fwupd,superm1/fwupd,hughsie/fwupd,vathpela/fwupd