usb gadget: use two luns on devices with two cards

  - ported from Codeworkx's smdk4210 patch (https://github.com/CyanogenMod/android_kernel_samsung_smdk4210/commit/7f87bd61861a7c6dfc74c6f113f4df655b8925ff)
  - allows to use both internal and external sd-card as mass-storage on devices with two cards

Change-Id: I3345097081bc1e306b8a63c0ae2b0edcccbff018