wic: extra-partition: introduce variable matching sourceparams

wic: extra-partition: introduce variable matching sourceparams

The current extra_partition plugin uses predictable variable names (the
configuration must predict the image partition label or uuid). This has two
inconveniences:

- an extra partition that has a specific label (or uuid) is doomed with the
  associated variable. The partition cannot be configured unless the label
  (or uuid) is changed.
- two partitions with same label from distincts .wks cannot have different
  content.

This allows the new suffix _name_%s to be appended to
IMAGE_EXTRA_PARTITION_FILES. This variable will be preferred whenever the
.wks specifies a name in the --sourceparams option.

For example:
  an image may define:
    IMAGE_EXTRA_PARTITION_FILES_name_foo = "bar.conf"
  and the .wks line have line:
    part --source extra_partition --sourceparams "name=foo"

Signed-off-by: Louis Rannou <louis.rannou@non.se.com>
Reviewed-by: Pierre-Loup GOSSE <pierre-loup.gosse@smile.fr>
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>