build(commitlint): make the scope optional

In all TF-A commit messages, the first line must comply to the
following format:

  type(scope): description

Although the conventional commits specification says that the scope
above is optional, we have made it mandatory in TF-A and the following
error message is printed if no scope is provided:

  scope may not be empty [scope-empty]

However, this can be too restrictive for some types of commits. For
example, it is typically hard to choose a scope for documentation
patches which modify several documents of different natures.

Lift this restriction in the tools and leave it up to the developer to
decide whether a scope is needed or not.

Change-Id: I9d35e7790fc3fa74651794216fe8db265ad09982
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>

.commitlintrc.js

@@ -68,7 +68,6 @@ module.exports = {
         "type-enum": [2, "always", types], /* Error */
 
         "scope-case": [2, "always", "lower-case"], /* Error */
-        "scope-empty": [2, "never"], /* Error */
         "scope-enum": [1, "always", scopes] /* Warning */
     },
 };