README: Add instructions for all different machines.

  o Machine configuration for the IGEPv2 AM/DM37x Processor Board
    MACHINE ?= "igep0020"

  o Machine configuration for the IGEP COM MODULE AM/DM37x Processor Board
    MACHINE ?= "igep0030"

  o Machine configuration for the IGEP COM AQUILA AM335x Processor Board
    MACHINE ?= "igep0033"

Signed-off-by: Enric Balletbo i Serra <eballetbo@gmail.com>