legal-info: fail trying to copy a non-existent license file

If a package's _LICENSE_FILES contains the name of a non-existent file,
the make process would continue and return true, unless the
non-existent file is the last listed.

Fix this wrong beaviour by failing with an error when any of the listed
files is missing.

Reported-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/pkg-generic.mk

@@ -508,9 +508,7 @@ ifeq ($(call qstrip,$$($(2)_LICENSE_FILES)),)
 	@$(call legal-license-nofiles,$$($(2)_RAWNAME))
 	@$(call legal-warning-pkg,$$($(2)_RAWNAME),cannot save license ($(2)_LICENSE_FILES not defined))
 else
-	@for F in $$($(2)_LICENSE_FILES); do \
-		$(call legal-license-file,$$($(2)_RAWNAME),$$$${F},$$($(2)_DIR)/$$$${F}); \
-		done
+	@$(foreach F,$($(2)_LICENSE_FILES),$(call legal-license-file,$$($(2)_RAWNAME),$(F),$$($(2)_DIR)/$(F))$$(sep))
 endif
 ifeq ($$($(2)_REDISTRIBUTE),YES)
 # Copy the source tarball (just hardlink if possible)