CMake: Make IDF_PATH available to Kconfiglib when not explicitly set.

CMake: Make IDF_PATH available to Kconfiglib when not explicitly set.

When IDF_PATH is not set by the user, cmake infers the same through
other means and sets it as an environment variable. However, some
sub-invocation of cmake may not see this variable as it is not set
in the parent. This change adds a custom command for exporting IDF_PATH
just before invoking ldgen so that IDF_PATH can be used in Kconfig
without any errors.