core/legal-info: save empty string when no license file

This is stored in the manifest file; it is perfectly possible to have
empty fields in there.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Luca Ceresoli <luca@lucaceresoli.net>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/pkg-generic.mk

@@ -779,7 +779,6 @@ endif
 ifneq ($$($(2)_LICENSE_FILES),)
 $(2)_MANIFEST_LICENSE_FILES = $$($(2)_LICENSE_FILES)
 endif
-$(2)_MANIFEST_LICENSE_FILES ?= not saved
 
 # We need to extract and patch a package to be able to retrieve its
 # license files (if any) and the list of patches applied to it (if