Rapper Curtis “50 Cent” Jackson is designing his own line of underwear for Frigo.  Courtesy G-Unit By Aria Hughes Rapper Curtis “50 Cent” Jackson is designing his own line of underwear for Frigo, a men’s underwear brand founded by Mathias Ingvarsson, the former president and European director of mattress brand Tempur-Pedic.  “My inspiration for this line came from the idea of the hierarchy of living, which is why you will see the crown as a constant throughout the designs,” said Jackson, who…