build: add APK package build capabilities

A new option called `USE_APK` is added which generated APK packages
(.apk) instead of OPKG packages (.ipk).

Some features like fstools `snapshot` command are not yet ported

Signed-off-by: Paul Spooren <mail@aparcar.org>

scripts/package-metadata.pl

@@ -373,7 +373,7 @@ sub and_condition($) {
 
 sub gen_condition ($) {
 	my $condition = shift;
-	# remove '!()', just as include/package-ipkg.mk does
+	# remove '!()', just as include/package-pack.mk does
 	$condition =~ s/[()!]//g;
 	return join("", map(and_condition($_), split('\|\|', $condition)));
 }