manifest: add support for selecting grub2.legacy

Older OS versions (RHEL 7) with older versions of grub2 don't support
BLS entries.  Setting NoBLS to true configures the bootloader with
traditional menu entries through the grub2.legacy osbuild stage.  This
requires specifying extra information for the OS to the pipeline:
version, product, and nick.

internal/image/live.go

@@ -26,6 +26,11 @@ type LiveImage struct {
 	Filename         string
 	Compression      string
 	PartTool         osbuild.PartTool
+
+	NoBLS     bool
+	OSProduct string
+	OSVersion string
+	OSNick    string
 }
 
 func NewLiveImage() *LiveImage {
@@ -47,6 +52,10 @@ func (img *LiveImage) InstantiateManifest(m *manifest.Manifest,
 	osPipeline.OSCustomizations = img.OSCustomizations
 	osPipeline.Environment = img.Environment
 	osPipeline.Workload = img.Workload
+	osPipeline.NoBLS = img.NoBLS
+	osPipeline.OSProduct = img.OSProduct
+	osPipeline.OSVersion = img.OSVersion
+	osPipeline.OSNick = img.OSNick
 
 	imagePipeline := manifest.NewRawImage(m, buildPipeline, osPipeline)
 	imagePipeline.PartTool = img.PartTool