wic: add freedom-u540 wic for sota/ostree
Wic file to be used with sota/ostree (meta-updater), as the source for
root needs to be provided by the otaimage plugin.
It can be maintained outside this layer, but having it maintained at the
official meta-riscv layer makes it a lot easier for users experimenting
with meta-updater.
Signed-off-by: Ricardo Salveti <ricardo@foundries.io>