boot: Replace occurrences of qualified name with full path

In many parts of this chapter the use of "qualified name" and
"unqualified name" was used to refer to a full path to a file (or just
the filename). This terminology is used more in networking terms (i.e.
fully qualified hostname) and is strange to use when referring to files
and directories in a filesystem. It leads to confusion for the reader,
which is why I replaced it throughout the chapter with the more commonly
used term "full path" where appropriate.

While here, remove some whitespaces at the end of sentences.

PR: 292300