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
Although they can be useful for debugging the codepath leading to
a problem, they are also confusing when it comes to the end user
messages:
```
{error #0} linux.c:406 device_get(): readlink of /sys/block/nvme0n1/device/driver failed: No such file or directory
{error #1} linux.c:406 device_get(): readlink of /sys/block/nvme0n1/device/driver failed: No such file or directory
{error #2} linux.c:406 device_get(): readlink of /sys/block/nvme0n1/device/driver failed: No such file or directory
{error #3} linux.c:406 device_get(): readlink of /sys/block/nvme0n1/device/driver failed: No such file or directory:
Error writing to file descriptor: No space left on device
```
Whereas if they weren't shown, that last message would have been plenty.
Additionally, if the user specified something invalid, do not autodetect the
ESP but return with a journal error. It seems wrong to ignore what the user
explicitly set and perhaps do something dangerous.
Alternative to https://github.com/hughsie/fwupd/pull/599
