Adjust u-boot-image-config, thanks to Vagrant Cascadian (Closes: #786369).

Two changes:
  - MX6_Cubox-i is now upstream, under a different name, with support
    for hummingboard-i1 and hummingboard-i2ex.
  - Wandboard now supports all three variants (Solo, Dual-Lite, and
    Quad), instead of just the Quad one.
  - PandaBoard: use u-boot.img instead of u-boot.bin