linux-yocto_3.14: fix: do not pollute variables of the main recipe

The restyling of the .bbappend was seemingly a good simplification but
building for qemux86 fails if the meta-handheld layer is included.

* ERROR: Function failed: Fetcher failure for URL: 'file://defconfig'.
* Unable to fetch URL from any source.

Go back to the previous syntax used for the older kernel versions and
fix the contamination of the variables for foreign machines once for all.

Signed-off-by: Andrea Adami <andrea.adami@gmail.com>