initscripts: add initscripts-functions-dev package

initscripts: add initscripts-functions-dev package

Add initscripts-functions-dev package to avoid initscripts package
being installed unexpected via the dependency of initscripts-dev package.

Before this patch, if we have 'dev-pkgs' in IMAGE_FEATURES, the initscripts
package would be installed once the initscripts-functions package is installed,
which is not what we need.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>