d/rules: Fix separate compilation of any/all packages (Closes: #1122141)

The concurrent merge of commit 87acf8fb and !2 (merged) lead to a d/rules that mixed the classic debhelper-based structure with the standard dh sequence.

That combination leads to subtle bugs. In particular compiling any and all binary packages separately breaks in various ways.

The merge also broke the override for dh_gencontrol and other dh tools.

This commit converts the change introduced in 87acf8fb to a standard dh override.