cmake: extensions: Change zephyr_file() argument parser prefix

Replace `FILE` with `ZFILE` as the prefix for `cmake_parse_arguments()`.
This is because `zephyr_file()` accepts `SUFFIX` as function argument,
and it should not be accessible as `FILE_SUFFIX` as it would conflict
with the CMake cache variable of the same name. Accessing the argument
as `ZFILE_SUFFIX` prevents the cached value from slipping into the
function scope.

Signed-off-by: Grzegorz Swiderski <grzegorz.swiderski@nordicsemi.no>