boot,bootloader: move RemoveKernelAssets and ExtractKernelAssets to bootloader's implementation

Different bootloaders might have different requirements about boot
asset extraction. Cleaner way is to move this implementation to each
bootloader implementation

Signed-off-by: Ondrej Kubik <ondrej.kubik@canonical.com>

bootloader/androidboot.go

@@ -26,6 +26,7 @@ import (
 	"github.com/snapcore/snapd/bootloader/androidbootenv"
 	"github.com/snapcore/snapd/dirs"
 	"github.com/snapcore/snapd/osutil"
+	"github.com/snapcore/snapd/snap"
 )
 
 type androidboot struct{}
@@ -75,3 +76,12 @@ func (a *androidboot) SetBootVars(values map[string]string) error {
 	}
 	return env.Save()
 }
+
+func (a *androidboot) ExtractKernelAssets(s *snap.Info, snapf snap.Container) error {
+	return nil
+
+}
+
+func (a *androidboot) RemoveKernelAssets(s snap.PlaceInfo) error {
+	return nil
+}