yocto-kernel: create open_user_file() wrapper function

yocto-kernel: create open_user_file() wrapper function

With the addition of custom kernels, we can no longer rely on a
hard-coded /files directory for BSPs - we need to be able to find the
user_config/patches files in a number of different directories.

We now hide the search inside a new open_user_file() function that
accomplishes the same thing as before but with a more flexible scope.

(From meta-yocto rev: 26a7032553e8d8691239368f0f994f948db06eed)

Signed-off-by: Tom Zanussi <tom.zanussi@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>