add Cubietruck to linux-armv7

Adds a packet called 'linux-armv7-cubietruck' to 'linux-armv7' to support the Cubieboard3/Cubietruck.

Changes:
* added _package-cubietruck() function
* added 'linux-armv7-cubietruck' to conflicts for each other packet
* bumped packet version 

Cubieboard 1 and 2 should work fine, too. But since i don't have those boards i couldn't test anything.

Things worth mentioning that aren't supported in 3.18:
* Bluetooth
* graphic output **
* NAND **

** added in 3.19-rc1