Use the ESP we booted from via LoaderDevicePartUUID

[Johan-Liebert1]

utils: Strip efivarfs attribute header and NUL when reading EFI vars

read_uefi_var() decoded the entire contents of the efivarfs file as
UTF-16LE.

efivarfs prepends a 4-byte little-endian u32 of the variable's attributes,
and EFI string variables are NUL-terminated.

As a result the returned string was wrapped in invisible characters and
incorrect string matches with UTF-8 encoded Strings

Skip the 4-byte attribute header before decoding and trim trailing
NUL and surrounding whitespace from the result.

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store: Select the booted ESP via LoaderDevicePartUUID

On dual-booted systems, walking up to the root disk(s) and picking the
first colocated ESP can return the wrong one, since there may be
multiple ESPs.

Instead, read the LoaderDevicePartUUID EFI variable set
by the boot loader and match it against the partuuid of the candidate
partitions, so we operate on the ESP we actually booted from.

The install paths still use find_first_colocated_esp(), since at that
point we are not booted into a bootc system and the EFI variable does
not describe the target system.

Fixes: #2554

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esp: Handle non-BLS bootloaders

Older versions of Grub, the one in c10s and c9s, don't fully implement
the BLS spec, which means we don't get the efivar
LoaderDevicePartUUID-4a67b082-0a4c-41cf-b6c7-440b29bb8c4f.

In such a case, go through all the ESPs and try to find bootc owned BLS
config file in ESP/loader/entries

[Johan-Liebert1]

I've tested this locally and it works fine. Unfortunately, adding integration tests for this is going to be a little bit hacky

[cgwalters]

Unfortunately, adding integration tests for this is going to be a little bit hacky

I think we can change our bcvk wrapper tooling to support provisioning block devices to the VM (bcvk supports it), then it's just a matter of adding varoius different ESP partitions, spread across various disks or so.