wic: Add --offset argument for partitions

Add support for an --offset argument when defining a partition. Many
SoCs require that boot partitions be located at specific offsets. Prior
to this argument, most WKS files were using the --align attribute to
specify the location of these fixed partitions but this is not ideal
because in the event that the partition couldn't be placed in the
specified location, wic would move it to the next sector with that
alignment, often preventing the device from booting. Unlike the --align
argument, wic will fail if a partition cannot be placed at the exact
offset specified with --offset.

Changes in V2:
* Fixed a small typo that prevented test_fixed_size_error from passing

(From OE-Core rev: 467f84e12b96bc977d57575023517dd6f8ef7f29)

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/lib/wic/plugins/imager/direct.py

@@ -428,6 +428,21 @@ class PartitionedImage():
                     # increase the offset so we actually start the partition on right alignment
                     self.offset += align_sectors
 
+            if part.offset is not None:
+                offset = (part.offset * 1024) // self.sector_size
+
+                if offset * self.sector_size != part.offset * 1024:
+                    raise WicError("Could not place %s%s at offset %dK with sector size %d" % (part.disk, self.numpart, part.offset, self.sector_size))
+
+                delta = offset - self.offset
+                if delta < 0:
+                    raise WicError("Could not place %s%s at offset %dK: next free sector is %d (delta: %d)" % (part.disk, self.numpart, part.offset, offset, delta))
+
+                logger.debug("Skipping %d sectors to place %s%s at offset %dK",
+                             delta, part.disk, self.numpart, part.offset)
+
+                self.offset = offset
+
             part.start = self.offset
             self.offset += part.size_sec