llext-edk: import data from build_info.yml and .config

Currently, the llext-edk.cmake script requires a number of variables to
be passed in from the main CMakeLists.txt file as arguments to be able
to customize the generated files.

To improve this rigid approach, the script is modified to read in the
following files in the build directory:

 * 'zephyr/.config', for the final set of Kconfig options used;
 * 'build_info.yml', for the cmake-related variables.

This is more flexible and also easier to maintain, as it doesn't require
manual changes to the main CMakelists.txt file when new variables need
to be referenced.

Signed-off-by: Luca Burelli <l.burelli@arduino.cc>