Update `--included-header` parameter

The `--included-header` was "mandatory" when using imgtool with firmware
images generated by the Zephyr build system and it was a source of
issues when it was forgotten. This removes `--included-header` and adds
a new parameter `--pad-header` with inverted semantics, to be used only
when a zeroed header is required to be added to the firmware image.

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/scripts/imgtool/image.py b/scripts/imgtool/image.py
index ea318f7..511c78c 100644
--- a/scripts/imgtool/image.py
+++ b/scripts/imgtool/image.py
@@ -66,7 +66,7 @@
 
 class Image():
     @classmethod
-    def load(cls, path, included_header=False, **kwargs):
+    def load(cls, path, pad_header=False, **kwargs):
         """Load an image from a given file"""
         ext = os.path.splitext(path)[1][1:].lower()
         if ext == INTEL_HEX_EXT:
@@ -78,7 +78,7 @@
         obj.payload, obj.base_addr = obj.load(path)
 
         # Add the image header if needed.
-        if not included_header and obj.header_size > 0:
+        if pad_header and obj.header_size > 0:
             obj.payload = (b'\000' * obj.header_size) + obj.payload
 
         obj.check()