efi: Operate on the ESP root to support non-EFI components

bootupd currently assumes that all managed content lives under the EFI/
subdirectory of the ESP. Platforms (such as the Raspberry Pi) place its
firmware blobs (device-tree files, GPU firmware, config.txt) directly in
the ESP root, outside EFI/. These files cannot be managed by bootupd today.

Widen the scope of all ESP operations (install, adopt, update, validate)
to work on the ESP root rather than just the EFI/ subdirectory.

Now, the file paths themselves determine their destination in the ESP. The
keys starting with EFI/ are written under the EFI/ subdirectory as before,
while all other files are placed directly at the ESP root.

Assisted-by: Cursor (Claude Opus 4)

Author: martinezjavier