bazel: Explicity specify pw_boot backend

Bug: 347998044
Change-Id: I20d3574c27c75dd1e483d9bf913d9f0aa73ca5bf
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/examples/+/217251
Commit-Queue: Auto-Submit <auto-submit@pigweed-service-accounts.iam.gserviceaccount.com>
Reviewed-by: Armando Montanez <amontanez@google.com>
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
Pigweed-Auto-Submit: Ted Pudlik <tpudlik@google.com>
Presubmit-Verified: CQ Bot Account <pigweed-scoped@luci-project-accounts.iam.gserviceaccount.com>
diff --git a/targets/transition.bzl b/targets/transition.bzl
index 353cca9..a443994 100644
--- a/targets/transition.bzl
+++ b/targets/transition.bzl
@@ -26,6 +26,7 @@
         "//command_line_option:platforms": "//targets/stm32f429i_disc1_stm32cube:platform",
         "@freertos//:freertos_config": "@pigweed//third_party/freertos:freertos_config",
         "@hal_driver//:hal_config": "@pigweed//targets/stm32f429i_disc1_stm32cube:hal_config",
+        "@pigweed//pw_boot:backend": "@pigweed//pw_boot_cortex_m",
         "@pigweed//pw_interrupt:backend": "@pigweed//pw_interrupt_cortex_m:context",
         "@pigweed//pw_log:backend": "@pigweed//pw_log_tokenized",
         "@pigweed//pw_log:backend_impl": "@pigweed//pw_log_tokenized:impl",
@@ -43,6 +44,7 @@
         "//command_line_option:platforms",
         "@freertos//:freertos_config",
         "@hal_driver//:hal_config",
+        "@pigweed//pw_boot:backend",
         "@pigweed//pw_interrupt:backend",
         "@pigweed//pw_log:backend_impl",
         "@pigweed//pw_log:backend",