[18.06] api: Change Platform field back to string (temporary workaround)

[tiborvass]

This partially reverts moby#37350

Although specs.Platform is desirable in the API, there is more work
to be done on helper functions, namely containerd's platforms.Parse
that assumes the default platform of the Go runtime.

That prevents a client to use the recommended Parse function to
retrieve a specs.Platform object.

With this change, no parsing is expected from the client.

Signed-off-by: Tibor Vass tibor@docker.com
(cherry picked from commit facad55)
Signed-off-by: Tibor Vass tibor@docker.com

Only a simple import path conflict:

diff --cc builder/builder-next/builder.go
index 5a82cddf44,b1d31a5225..0000000000
--- a/builder/builder-next/builder.go
+++ b/builder/builder-next/builder.go
@@@ -14,7 -13,8 +14,12 @@@ import
        "github.com/docker/docker/api/types/backend"
        "github.com/docker/docker/builder"
        "github.com/docker/docker/daemon/images"
++<<<<<<< HEAD
 +      "github.com/docker/docker/pkg/jsonmessage"
++=======
+       "github.com/docker/docker/pkg/streamformatter"
+       "github.com/docker/docker/pkg/system"
++>>>>>>> facad55744... api: Change Platform field back to string (temporary workaround)
        controlapi "github.com/moby/buildkit/api/services/control"
        "github.com/moby/buildkit/control"
        "github.com/moby/buildkit/identity"

[thaJeztah]

cherry pick of moby#37381

[tiborvass]

Closing in favor of #7 for conflict reasons