Boot: Add version number from command line

- Add an optional command line argument that can be used to set an
image's version number at build-time (using
-DIMAGE_VERSION=maj.min.rev+build) so that the Cmake command within
bl2/ext/mcuboot/MCUBoot.cmake doesn't need to be edited each time the
specified version number needs to be changed
- If the version number is not defined in the command line, use a
default value and automatically increment it in the next build

Signed-off-by: Oliver Swede <oli.swede@arm.com>
Change-Id: I3ff44ffb8219d7ea1a441d918cceb525e7af60f2
diff --git a/bl2/ext/mcuboot/scripts/imgtool/image.py b/bl2/ext/mcuboot/scripts/imgtool/image.py
index f8309b3..9731844 100644
--- a/bl2/ext/mcuboot/scripts/imgtool/image.py
+++ b/bl2/ext/mcuboot/scripts/imgtool/image.py
@@ -79,8 +79,8 @@
         obj.check()
         return obj
 
-    def __init__(self, version=None, header_size=IMAGE_HEADER_SIZE, pad=0):
-        self.version = version or versmod.decode_version("0")
+    def __init__(self, version, header_size=IMAGE_HEADER_SIZE, pad=0):
+        self.version = version
         self.header_size = header_size or IMAGE_HEADER_SIZE
         self.pad = pad
 
@@ -182,4 +182,4 @@
         pbytes  = b'\xff' * padding
         pbytes += b'\xff' * (tsize - len(boot_magic))
         pbytes += boot_magic
-        self.payload += pbytes
+        self.payload += pbytes
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