distro/fedora: don't return early from FIPS check in checkOptions()

When the check was first added [1] it was the last in the function and
was written to immediately return.  New checks were added after the
check, without changing the early return, which means invalid
configurations were not caught.

Append to warnings instead of returning.

[1] 665a128

pkg/distro/fedora/imagetype.go

@@ -290,6 +290,8 @@ func (t *imageType) Manifest(bp *blueprint.Blueprint,
 func (t *imageType) checkOptions(bp *blueprint.Blueprint, options distro.ImageOptions) ([]string, error) {
 	customizations := bp.Customizations
 
+	var warnings []string
+
 	if !t.rpmOstree && options.OSTree != nil {
 		return nil, fmt.Errorf("OSTree is not supported for %q", t.Name())
 	}
@@ -448,8 +450,7 @@ func (t *imageType) checkOptions(bp *blueprint.Blueprint, options distro.ImageOp
 	}
 
 	if customizations.GetFIPS() && !common.IsBuildHostFIPSEnabled() {
-		w := fmt.Sprintln(common.FIPSEnabledImageWarning)
-		return []string{w}, nil
+		warnings = append(warnings, fmt.Sprintln(common.FIPSEnabledImageWarning))
 	}
 
 	instCust, err := customizations.GetInstaller()
@@ -474,5 +475,5 @@ func (t *imageType) checkOptions(bp *blueprint.Blueprint, options distro.ImageOp
 		}
 	}
 
-	return nil, nil
+	return warnings, nil
 }