This should help in rare hardware-specific situations where the root is
not automatically detected properly.
We search using a marker file. This should help some weird UEFI setups
where the root is set to `(hd0,msdos2)` by default.
Defaulting to `(hd0)` by looking for the ESP **will break themeing**. It
is unclear why, but files in `(hd0,msdos2)` are not all present as they
should be.
This also fixes an issue introduced with cb5c4fcd3c
where rEFInd stopped booting in many cases. This is because it ended up
using (hd0) rather than using the `search` which was happening
beforehand, which in turn uses (hd0,msdos2), which is the ESP.
Putting back the `search` here fixes that.
20b023b5ea