commands: check if flag is set instead of using flagutil.Tristate

Fixes --pull and --no-cache without argument

Signed-off-by: Tibor Vass <tibor@docker.com>

util/flagutil/flagutil.go

@@ -1,36 +0,0 @@
-package flagutil
-
-import "strconv"
-
-type tristate struct {
-	opt *bool
-}
-
-// Tristate is a tri-state boolean flag type.
-// It can be set, but not unset.
-func Tristate(opt *bool) tristate {
-	return tristate{opt}
-}
-
-func (t tristate) Type() string {
-	return "tristate"
-}
-
-func (t tristate) String() string {
-	if t.opt == nil {
-		return "(unset)"
-	}
-	if *t.opt {
-		return "true"
-	}
-	return "false"
-}
-
-func (t tristate) Set(s string) error {
-	b, err := strconv.ParseBool(s)
-	if err != nil {
-		return err
-	}
-	t.opt = &b
-	return nil
-}