distro/fedora: image type should be more specific than arch

Currently an image type could override the boot loader in the architecture, but we should
not allow an image type to select a bootloader type the architecture does not support. So
only in case the architecture supports hybrid boot do we allow the image type to select
one of the two types.

This enables UEFI on AMIs for aarch64.

internal/distro/fedora/distro.go

@@ -649,7 +649,9 @@ func (t *imageType) Exports() []string {
 func (t *imageType) getBootType() distro.BootType {
 	bootType := t.arch.bootType
 	if t.bootType != distro.UnsetBootType {
-		bootType = t.bootType
+		if bootType == distro.HybridBootType {
+			bootType = t.bootType
+		}
 	}
 	return bootType
 }