Allow Entityreference for Add to Cart field formatter

Problem/Motivation

There are simple, off-the-shelf solutions like cer that work with entityreference, but don't work with product_reference. It's kind of backwards to require that these modules be hacked to use product_reference, instead of adopting/embracing entityreference more completely.

Proposed resolution

There's the attached patch which has the negative UX consequence of allowing the user to select add_to_cart_form as the field formatter for entityreference field instances that don't actually target product entities. The way around this would be to add a form_alter hook for field_ui_display_overview_form to inject a call that queries the field info API and asks if the entityreference field instance actually is targeting entities of type product and modifies the options accordingly.

Remaining tasks

If this is found to be a good idea, then what remains is to add the alter implementation to the patch. If not, then perhaps it would be good to push this up to core field module and ask for the option to supply a callable that determines if a formatter can be used, rather than hard-coding the entity names.

User interface changes

You would get to use entityreference as well as or instead of product_reference, at least as far as add_to_cart_form goes.

API changes

Anyone expecting the entity id field to be named 'product_id' would be SOL unless we make a point of supplying control flow to mask the difference, since entityreference calls this field 'target_id'.