Add experimental way to set a target platform when building

Add an --experimental-platform flag to the do command to allow
overriding the default auto-detected build platform until we find the
time to think about the definitive multi-platform builds UX

Signed-off-by: Marcos Lilljedahl <marcosnils@gmail.com>

plancontext/platform.go

@@ -25,7 +25,3 @@ func (c *platformContext) SetString(platform string) error {
 func (c *platformContext) Set(p specs.Platform) {
 	c.platform = &p
 }
-
-func (c *platformContext) IsSet() bool {
-	return c.platform != nil
-}