sanitycheck: fix priority of --extra-args=CONFIG_ over testcase.yaml

sanitycheck takes any "extra_config" list found in the testcase.yaml
file and generates an "overlay" file from it. This file is placed in
the per-test build directory and passed to cmake/kconfig.cmake through a
-DOVERLAY_CONF= option set in the (also) generated sanity-out/Makefile.

This commit moves this generated config overlay to a subdirectory one
level down from the build directory, otherwise kconfig.cmake picks it
up *twice*: once from the -DOVERLAY_CONF= option already mentioned above
and a second time because kconfig.cmake scans the build directory and
blindly picks up ALL files ending with .conf[*]. The second pickup is
problematic because kconfig.cmake currently gives it the top precedence,
even higher than anything the user espressed with --extra-args=CONFIG_*

Here's a quick and simple demonstration of the issue fixed by this
commit:

  cd $ZEPHYR_BASE/samples/net/sockets/net_mgmt/
  sanitycheck -T. -p qemu_x86 -b -v # --extra-args=CONFIG_USERSPACE=y|n
  grep CONFIG_USERSPACE $(find sanity-out/ -name .config)

  .net_mgmt.kernelmode/zephyr/.config: # CONFIG_USERSPACE is not set
    .net_mgmt.usermode/zephyr/.config: CONFIG_USERSPACE=y

  grep 'Merged configuration' $(find sanity-out/ -name build.log)

Without this commit, attemps to override anything with
--extra-args=CONFIG_ are silently dropped on the floor.

For more background this issue was found while using the recipe in
commit message 4afcc0f8af

[*] picking up all .conf files is debatable but a much bigger debate
with backward compatibility implications. This small fix makes
absolutely zero difference to anyone or anything not using sanitycheck.

Signed-off-by: Marc Herbert <marc.herbert@intel.com>

cmake/kconfig.cmake

@@ -94,7 +94,7 @@ get_cmake_property(cache_variable_names CACHE_VARIABLES)
 foreach (name ${cache_variable_names})
   if("${name}" MATCHES "^CONFIG_")
     # When a cache variable starts with 'CONFIG_', it is assumed to be
-    # a CLI Kconfig symbol assignment.
+    # a Kconfig symbol assignment from the CMake command line.
     set(EXTRA_KCONFIG_OPTIONS
       "${EXTRA_KCONFIG_OPTIONS}\n${name}=${${name}}"
       )

scripts/sanitycheck

@@ -1334,7 +1334,8 @@ class MakeGenerator:
 
         if len(ti.test.extra_configs) > 0 or options.coverage:
             args.append("OVERLAY_CONFIG=\"%s %s\"" %(overlays,
-                        os.path.join(ti.outdir, "overlay.conf")))
+                        os.path.join(ti.outdir,
+                                     "sanitycheck", "testcase_extra.conf")))
 
         if ti.test.type == "unit" and options.enable_coverage:
             args.append("COVERAGE=1")
@@ -1885,8 +1886,13 @@ class TestInstance:
         return build_only
 
     def create_overlay(self, platform):
-        file = os.path.join(self.outdir, "overlay.conf")
-        os.makedirs(self.outdir, exist_ok=True)
+        # Create this in a "sanitycheck/" subdirectory otherwise this
+        # will pass this overlay to kconfig.py *twice* and kconfig.cmake
+        # will silently give that second time precedence over any
+        # --extra-args=CONFIG_*
+        subdir = os.path.join(self.outdir, "sanitycheck")
+        os.makedirs(subdir, exist_ok=True)
+        file = os.path.join(subdir, "testcase_extra.conf")
         with open(file, "w") as f:
             content = ""