Improved patchfiles handling for the case a given file does not exist

- Till now we didn't checked if a patch does really exist, so you will
  mostly just notice if you search the build log for files that were
  not found.
- By now we will issue a file not found warning for that case so
  problems related to not existing patch files will be more obvious.

lib/functions.in

@@ -764,19 +764,24 @@ apply_patchfiles() {
 			x="$base/download/mirror/${x:0:1}/$x"
 		fi
 
-		echo "Apply patch $x ..."
+		# Apply the patch if the file does exist or issue a warning
+		if [ -f "$x" ] ; then
+			echo "Apply patch $x ..."
 
-		if [[ $x = *.bz2 ]] ; then
-			patch_file=`mktemp` ; patch_del=1
-			bzcat $x > $patch_file
-		else
-			patch_file=$x ; patch_del=0
-		fi
+			if [[ $x = *.bz2 ]] ; then
+				patch_file=`mktemp` ; patch_del=1
+				bzcat $x > $patch_file
+			else
+				patch_file=$x ; patch_del=0
+			fi
 
-		$filter $patch_file | patch $patchopt
+			$filter $patch_file | patch $patchopt
 
-		[ $patch_del = 1 ] && rm $patch_file
-		eval "$hook"
+			[ $patch_del = 1 ] && rm $patch_file
+			eval "$hook"
+		else
+			echo_warning "Unable to apply patch: $x (File not found!)"
+		fi
         done
 }