rhel84/grub2: set saved_entry

Explicitly set the kernel to boot into.

Also change the blueprint/kernenl handling:

Rather than only falling back to the default kernel name for
getting the package list, let GetKernel() always return the
correct result so we can rely on this being consistent.

Signed-off-by: Tom Gundersen <teg@jklm.no>

internal/osbuild/grub2_stage.go

@@ -16,6 +16,7 @@ type GRUB2StageOptions struct {
 	KernelOptions      string     `json:"kernel_opts,omitempty"`
 	Legacy             string     `json:"legacy,omitempty"`
 	UEFI               *GRUB2UEFI `json:"uefi,omitempty"`
+	SavedEntry         string     `json:"saved_entry,omitempty"`
 }
 
 type GRUB2UEFI struct {