Firsly, that HSI isn't expected for embedded devices and then secondary how we
require SecureBoot to be available for HSI:1
At the moment we get a runtime failure if it is disabled. Making SB a part of
`HSI:1` makes this requiremnt explicit and prevents us getting `HSI:2!` if it
is not available.
If the measurements are missing but it's a UEFI system, it's a good indication
that the user has secure boot turned off.
Notify the user on the UEFI device through a non-fatal `UpdateMessage`
To accomplish this, move fu-uefi-vars into the plugin library for other plugins to use
