Per discussion of #2513, Ubuntu Core was going to use UEFI removable
path as default esp path, and it needs to change some parts around
getting the esp path and searching the shim app path. Also, a new option
"FallbacktoRemovablePath" is added into uefi.conf to be applied in this
case, and it will be false by default.
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
