refactor(ulp): provide a ulp_project.cmake subproject wrapper API

Build ULP full subprojects through a dedicated entry file,
components/ulp/cmake/ulp_project.cmake, that wraps tools/cmakev2/idf.cmake
and layers a small ULP API on top, mirroring the cmakev2 layering:

  ulp_project_init      like idf_project_init (init, detect the ULP type,
                        reset the compile/link options inherited from the app)
  ulp_build_executable  like idf_build_executable, plus the embeddable
                        .bin/.h/.ld artifacts
  ulp_project_default   like idf_project_default (single-executable case)

A child project now includes this one file instead of idf.cmake and calls
these helpers directly, so idf_build_executable is used as-is for the
multi-binary case and the module-path indirection (include(IDFULPProject)
resolved via -DCMAKE_MODULE_PATH) is gone.

As a result:
- IDFULPProjectv2.cmake is removed; its setup moves into the wrapper.
- IDFULPProject.cmake becomes the CMake v1-only entry point.
- The ULP component no longer registers a POST_ELF callback; the binary
  artifacts are produced by ulp_build_executable.
- The v2 full-subproject examples (lp_core, riscv, fsm, multi_binary,
  combined) are updated to the new API.
- The ULP subproject API is documented in build-system-v2.rst.

Also fix a latent bug this exercises: idf_build_library emitted linker
scripts as "-T <name>" relying on a following "-L" search directory. GNU
ld only searches -L directories that precede -T, so the direct
esp32ulp-elf-ld link used for ULP FSM failed to open the script. Emit the
absolute path instead, matching what the CMake v1 ULP build already does.

Finally, replace the parent-argument bypass loop that used to live in
IDFULPProjectv2.cmake with --no-warn-unused-cli on the child configure,
and stop passing the unused IDF_PARENT_BUILD_DIR.

Signed-off-by: Frantisek Hrbata <frantisek.hrbata@espressif.com>

tools/cmakev2/build.cmake

@@ -599,12 +599,14 @@ function(idf_build_library library)
     # seen before any section-placement script references them.
     foreach(script IN LISTS memory_scripts other_scripts)
         get_filename_component(script_dir "${script}" DIRECTORY)
-        # Add linker script directory to the linker search path.
+        # Add the linker script directory to the linker search path so INCLUDE
+        # directives inside a script can resolve sibling scripts by name.
         target_link_directories("${library}" INTERFACE "${script_dir}")
-        # Add linker script to link. Use the full path because direct ld
-        # resolves -T against only the -L directories that came before it, while
-        # CMake may emit link options before link directories. Keep -L above so
-        # linker scripts can still INCLUDE siblings by name.
+        # Add the linker script to the link by absolute path. Passing the full
+        # path (rather than a bare name resolved via -L) keeps this working for
+        # direct linker drivers such as the ULP FSM esp32ulp-elf-ld link, where
+        # the -L search directories follow the -T options on the command line
+        # and GNU ld therefore does not use them to locate the -T script.
         # Regarding the usage of SHELL, see
         # https://cmake.org/cmake/help/latest/command/target_link_options.html#option-de-duplication
         # Quote the path: SHELL: strings are re-split on spaces, so an unquoted