Show Clover's boot log again; version 5.0.7

The boot log was decoded with String(data:encoding: .ascii), which returns
nil if any byte is over 0x7F, so a Clover log with one such character
showed as missing (and Clover boot-disk detection from SelfDevicePath
failed). Decode as UTF-8, falling back to Latin-1, and drop trailing NULs.
Fixes #257.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>