bazel: Explicitly set pw_log backend

Bug: 347998044
Change-Id: I08712ff30a12c254623c4e617666e7d0ffa1040e
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/quickstart/bazel/+/218215
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
Reviewed-by: Taylor Cramer <cramertj@google.com>
Pigweed-Auto-Submit: Ted Pudlik <tpudlik@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed-service-accounts.iam.gserviceaccount.com>
diff --git a/echo.bzl b/echo.bzl
index b162e0b..66ef62f 100644
--- a/echo.bzl
+++ b/echo.bzl
@@ -19,6 +19,8 @@
     return {
         "//command_line_option:platforms": "//targets:stm32",
         "@pigweed//pw_boot:backend": "@pigweed//pw_boot_cortex_m",
+        "@pigweed//pw_log:backend": "@pigweed//pw_log_basic",
+        "@pigweed//pw_log:backend_impl": "@pigweed//pw_log_basic:impl",
     }
 
 _stm32_transition = transition(
@@ -27,6 +29,8 @@
     outputs = [
         "//command_line_option:platforms",
         "@pigweed//pw_boot:backend",
+        "@pigweed//pw_log:backend",
+        "@pigweed//pw_log:backend_impl",
     ],
 )