pw_build: Switch to pw_* target types

This change replaces the implicit overrides of the default GN target
types in BUILDCONFIG.gn with explicit pw_* versions. This is done to
allow Pigweed to build within larger GN projects without modifying their
BUILDCONFIG files.

Change-Id: I1d1e41d5e9bde8fe076f7031f3c2b98b9630ff70
diff --git a/pw_sys_io_baremetal_lm3s6965evb/BUILD.gn b/pw_sys_io_baremetal_lm3s6965evb/BUILD.gn
index d032612..47219ae 100644
--- a/pw_sys_io_baremetal_lm3s6965evb/BUILD.gn
+++ b/pw_sys_io_baremetal_lm3s6965evb/BUILD.gn
@@ -12,12 +12,13 @@
 # License for the specific language governing permissions and limitations under
 # the License.
 
+import("$dir_pw_build/target_types.gni")
 import("$dir_pw_docgen/docs.gni")
 
 # This if statement allows docs to always build even if the target isn't
 # compatible with this backend.
 if (dir_pw_sys_io_backend == dir_pw_sys_io_baremetal_lm3s6965evb) {
-  source_set("pw_sys_io_baremetal_lm3s6965evb") {
+  pw_source_set("pw_sys_io_baremetal_lm3s6965evb") {
     public_deps = [ "$dir_pw_boot_armv7m" ]
     deps = [
       "$dir_pw_preprocessor",