genimage: allow non-sparse image-less partitions not in partition table

Previously, partitions not included in a partition table were required
to always reference an image.

However, it can be useful to define such partitions without an image,
for example, for "barebox-env" or "barebox-state" regions at fixed
offsets in legacy MBR setups. These partitions are typically populated at
runtime and therefore do not need an image.

To ensure that flashing a sparse image variant results in a well-defined
state, these partitions should be explicitly filled with zeros to
overwrite any existing data.

So allow partitions not present in the partition table, without an
image, to be filled with zeros, e.g.:

  partition barebox-environment {
    size = 128K
    in-partition-table = false
    sparse = false
  }

Signed-off-by: Bastian Krause <[email protected]>

Author: Bastian-Krause

README.rst

@@ -130,8 +130,10 @@ Partition options:
 :sparse:		If true (the default) 'holes' in the input images are preserved
 			and the remaining free space in the partition is also a 'hole'.
 			If false, the 'holes' in the input image are explicitly
-			filled with zeros when the image is copied. If ``fill``
-			is specified as well then the remaining free space is
+			filled with zeros when the image is copied.
+			If the partition has no image, the whole partition content is a
+			'hole'.
+			If ``fill`` is specified as well then the remaining free space is
 			also filled with zeros.
 :autoresize:		Boolean specifying that the partition should be resized
 			automatically. For UBI volumes this means that the

genimage.c

@@ -831,7 +831,11 @@ int main(int argc, char *argv[])
 			struct image *child;
 
 			if (!part->image) {
-				if (part->in_partition_table)
+				/* allow image-less partitions that are present
+				 * in the partition table and partitions that
+				 * should be filled with zeros (sparse = false)
+				 */
+				if (part->in_partition_table || !part->sparse)
 					continue;
 				image_error(image, "no input file given\n");
 				ret = -EINVAL;