targets: move early_boot.c to boot.cc

Alters the existing targets to use a boot.cc file instead of the
existing early_boot.c.

The name was updated to reflect the fact that it now contains a
pw_boot_PostMain() hook which is after early boot.

C++ was selected over C to give more language flexibility.

Change-Id: I18ca0b8720bf083d4f644f1ca994e8fd7adc89b6
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/16702
Commit-Queue: Ewout van Bekkum <ewout@google.com>
Reviewed-by: Armando Montanez <amontanez@google.com>
Reviewed-by: Keir Mierle <keir@google.com>
diff --git a/pw_sys_io_baremetal_stm32f429/BUILD.gn b/pw_sys_io_baremetal_stm32f429/BUILD.gn
index 9df8bbf..334bed0 100644
--- a/pw_sys_io_baremetal_stm32f429/BUILD.gn
+++ b/pw_sys_io_baremetal_stm32f429/BUILD.gn
@@ -24,9 +24,11 @@
 pw_source_set("pw_sys_io_baremetal_stm32f429") {
   public_configs = [ ":default_config" ]
   public = [ "public/pw_sys_io_baremetal_stm32f429/init.h" ]
-  public_deps = [ "$dir_pw_boot_armv7m" ]
-  deps = [
+  public_deps = [
+    "$dir_pw_boot_armv7m",
     "$dir_pw_preprocessor",
+  ]
+  deps = [
     "$dir_pw_sys_io:default_putget_bytes",
     "$dir_pw_sys_io:facade",
   ]