linux: change kernel branch
linux: change kernel branch

Use 'technux-dev' instead of 'technux-v3.14-dev' since that branch has
gone end of life.

Also ensure to always set LINUX_VERSION_EXTENSION since yocto has
already set this variable in an earlier stage.

Change-Id: Ibd0a83f193a1f19cb90c6df7d1d46a523bfee561

Technux/meta-technux,Technux/meta-technux