build(hooks): allow hooks to skip Commitizen

Adds a conditional check in the `prepare-commit-msg` commit hook that
reads the `tf-a.disableCommitizen` Git configuration option, and
does not execute Commitizen if it is found.

To skip Commitizen, run:

    git config tf-a.disableCommitizen true

Change-Id: Ic8967f6f42bf3555df09b57096044fb99438d4d4
Signed-off-by: Chris Kay <chris.kay@arm.com>

.husky/prepare-commit-msg

@@ -3,4 +3,6 @@
 # shellcheck source=./_/husky.sh
 . "$(dirname "$0")/_/husky.sh"
 
+if ! git config --get tf-a.disableCommitizen > /dev/null; then
     "$(dirname "$0")/prepare-commit-msg.cz" "$@"
+fi