plymouth: allow disabling the dracut dep

Add a 'initrd' PACKAGECONFIG to disable emission of the plymouth-initrd
package which includes initramfs files for use by dracut. Disabling this will
avoid the runtime dependency on dracut, which is useful if one doesn't need or
want dracut and doesn't want to pull in meta-initramfs.

Signed-off-by: Christopher Larson <chris_larson@mentor.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
(cherry picked from commit 240e2ebe4371f15fad37b31bd3db83aa0ab42522)
Signed-off-by: Armin Kuster <akuster808@gmail.com>