package/linux-firmware: different iwlwifi3160/7260 firmwares for different kernels

The firmwares to use for iwlwifi 3160/7260 are different, depending on which
version of the Linux kernel is being used:
  - rev. 7 is for linux 3.10 through 3.12 (both included)
  - rev. 8 is for linux 3.13 onward

Add a config choice to select the appropriate version.

(See cset a0a6eeb in the linux-firmware repository for the details.)

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>