linux: add fbtft kernel extension

Tested with RaspberryPi B+ and PiTFT Mini Kit - 320x240 2.8" TFT
(see [1] and [2]) and the following target configuration changes:
- cmdline.txt: add 'fbcon=map:10 fbcon=font:VGA8x8'
- add /etc/modules-load.d/fbtft.conf with 'fbtft_device'
- add /etc/modprobe.d/00-fbtft.conf with 'options fbtft_device name=adafruit28 rotate=90 gpios=dc:25'

[1] http://h65951.serverkompetenz.net/PeterSeiderer/upload/PiTFT_2_8_ct/Image9893.jpg
[2] http://h65951.serverkompetenz.net/PeterSeiderer/upload/PiTFT_2_8_ct/Image9897.jpg



[Thomas:
  - Rename prompt of the Linux extension to "FB TFT drivers"
  - Remove the full name of the kernel config options in the help
    text. Giving their CONFIG_<foo> name is enough.
  - Remove the mention of CONFIG_SPI_BCM2708, since this makes the
    description RaspberryPi specific, while these drivers can work
    with any SPI controller.
  - Refactor the code in linux-ext-fbtft.mk to avoid duplication
    between the < 3.15 and >= 3.15 cases.
  - Make the fbtft package a promptless package, since there is no
    point in selecting only this package, without the kernel
    extension.
  - Change the license to GPLv2, since it's kernel code.]

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>