modutils-initscripts: Use depmod -a when modules.dep is empty

To correctly create the modules.dep when it is empty, we need to call depmod
with '-a' instead of '-Ae' since the quick option uses the mtime to decide
whether or not it needs to update. This way we can guarantee that it will be
recreated completely and not only when adding new modules.

Signed-off-by: Diego Sueiro <diego.sueiro@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 9292e905)
Signed-off-by: Steve Sakoman <steve@sakoman.com>