Add compiled fex file into the image (just for OLinuXino A10S now)

The fresh images I created for Cubieboard and OLinuXino were not able to boot because of missing script.bin file. I'm not sure if I miss something, but the only way to go was to put script.bin to the FAT partition by hand.
These commits does this automatically. First commit just fixes some issue with build of the native part of sunxi-tools. The main commit adds new recipe for "building" fex files. The fex files are taken from linux-sunxi/sunxi-borads git.
Adding of a new machine should be easy:
In file: recipes-bsp/sunxi-board-fex/sunxi-board-fex.bb
- Add SUNXI_FEX_FILE_\<your_machine\> = "sys_config/path/to/fex/in/sunxi-board-git"
- Add your_machine to COMPATIBLE_MACHINE

In file classes/sdcard_image-a10.bbclass:
- Create new variable for your_machine according to IMAGE_DEPENDS_a10-sdimg_olinuxino-a10s (ie. IMAGE_DEPENDS_a10-sdimg_\<your_machine\>).

The goal would be to do all these steps automatically - ie. create strict dependency (IMAGE_DEPENDS without machine modificator) on sunxi-board-fex in a10-sdimage. But this needs to adapt sunxi-board-fex for all available machines in this layer. I'm not able to do this as I'm not able to test all of these boards.