imgtool: add erased-val cli option

Add `-R`/`--erased-val` CLI option when creating images, which accepts
a value to be used for padding. This is useful when creating images
that run in devices that have flash erased to some value different
from 0xff (like STM32L0x and STM32L1x that have 0s).

Signed-off-by: Fabio Utzig <utzig@apache.org>

scripts/imgtool/image.py

@@ -113,7 +113,7 @@ class Image():
     def __init__(self, version=None, header_size=IMAGE_HEADER_SIZE,
                  pad_header=False, pad=False, align=1, slot_size=0,
                  max_sectors=DEFAULT_MAX_SECTORS, overwrite_only=False,
-                 endian="little", load_addr=0):
+                 endian="little", load_addr=0, erased_val=0xff):
         self.version = version or versmod.decode_version("0")
         self.header_size = header_size
         self.pad_header = pad_header
@@ -125,6 +125,7 @@ class Image():
         self.endian = endian
         self.base_addr = None
         self.load_addr = 0 if load_addr is None else load_addr
+        self.erased_val = 0xff if erased_val is None else int(erased_val)
         self.payload = []
         self.enckey = None
 
@@ -160,7 +161,8 @@ class Image():
             if self.base_addr:
                 # Adjust base_addr for new header
                 self.base_addr -= self.header_size
-            self.payload = (b'\000' * self.header_size) + self.payload
+            self.payload = bytes([self.erased_val] * self.header_size) + \
+                self.payload
 
         self.check()
 
@@ -347,8 +349,8 @@ class Image():
         tsize = self._trailer_size(self.align, self.max_sectors,
                                    self.overwrite_only, self.enckey)
         padding = size - (len(self.payload) + tsize)
-        pbytes = b'\xff' * padding
-        pbytes += b'\xff' * (tsize - len(boot_magic))
+        pbytes = bytes([self.erased_val] * padding)
+        pbytes += bytes([self.erased_val] * (tsize - len(boot_magic)))
         pbytes += boot_magic
         self.payload += pbytes
 

scripts/imgtool/main.py

@@ -182,6 +182,9 @@ class BasedIntParamType(click.ParamType):
 
 @click.argument('outfile')
 @click.argument('infile')
+@click.option('-R', '--erased-val', type=click.Choice(['0', '0xff']),
+              required=False,
+              help='The value that is read back from erased flash.')
 @click.option('-x', '--hex-addr', type=BasedIntParamType(), required=False,
               help='Adjust address in hex output file.')
 @click.option('-L', '--load-addr', type=BasedIntParamType(), required=False,
@@ -214,12 +217,12 @@ class BasedIntParamType(click.ParamType):
                .hex extension, otherwise binary format is used''')
 def sign(key, align, version, header_size, pad_header, slot_size, pad,
          max_sectors, overwrite_only, endian, encrypt, infile, outfile,
-         dependencies, load_addr, hex_addr):
+         dependencies, load_addr, hex_addr, erased_val):
     img = image.Image(version=decode_version(version), header_size=header_size,
                       pad_header=pad_header, pad=pad, align=int(align),
                       slot_size=slot_size, max_sectors=max_sectors,
                       overwrite_only=overwrite_only, endian=endian,
-                      load_addr=load_addr)
+                      load_addr=load_addr, erased_val=erased_val)
     img.load(infile)
     key = load_key(key) if key else None
     enckey = load_key(encrypt) if encrypt else None