linux: bump rpi-kernel to 3.12.26

meta-raspberrypi has bumped kernel version from 3.12.21 to 3.12.26.
Update bbappend to handle this change.

Change-Id: I039218d9dccb7316c1b6e98a48d47e7187e07e37
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