[FIX] mrp_sale: compute unit price of kit

When the unit price of a kit is computed in anglo saxon accounting, the
average price of kit parts is not multiplied by the quantity of this
parts in the bom.

E.g.: a kit of 2 prod1 and 1 prod2, when the price is computed, it only
take into account the cost of 1 prod1 and 1 prod2 to compute the kit
cost instead of 2 prod1 and 1 prod2

opw-786268