cli: Rename user "config" to user "preference"

Commit 754d6d6 ("cli: Generalize FASTPATH_LIBRARY into a key-value
store") and commit ef6cf61 ("cli: Add "config set" and "config get"
commands") introduced a general purpose "user config" key-value store
together with a command line interface ("config get"/"config set") to
access and modify the values.

But the term "config" is massively overloaded and confusing to users. So
let's rename to "preferences" instead. The cli noun is now called
"preference" and the peristent store file is renamed from
~/.fastpathconfig to ~/.fastpathpreferences.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>