Update bootloader image headers before debugging or uploading via debug tools

This approach is less intrusive than merging the entire application into one binary
implemented in #006d64e8b268e479703a0aac7eed8bef1ebea587

In this implementation we safely copy the required bootloader binary to the
build directory, adjust the headers via esptoolpy and the "merge_bin" command
according to --flash-size and --flash-mode arguments.

Resolves #872

platform.py

@@ -261,23 +261,16 @@ class Espressif32Platform(PlatformBase):
         if any(ignore_conds):
             return
 
-        merged_firmware = build_extra_data.get("merged_firmware", False)
-        load_cmds = []
-        if not merged_firmware:
-            load_cmds.extend([
-                'monitor program_esp "{{{path}}}" {offset} verify'.format(
-                    path=to_unix_path(item["path"]), offset=item["offset"]
-                )
-                for item in flash_images
-            ])
-
+        load_cmds = [
+            'monitor program_esp "{{{path}}}" {offset} verify'.format(
+                path=to_unix_path(item["path"]), offset=item["offset"]
+            )
+            for item in flash_images
+        ]
         load_cmds.append(
             'monitor program_esp "{%s.bin}" %s verify'
             % (
-                to_unix_path(
-                    debug_config.build_data["prog_path"][:-4]
-                    + ("_merged" if merged_firmware else "")
-                ),
+                to_unix_path(debug_config.build_data["prog_path"][:-4]),
                 build_extra_data.get("application_offset", "0x10000"),
             )
         )