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>

rules.mk

@@ -249,6 +249,8 @@ HOST_CFLAGS:=-O2 $(HOST_CPPFLAGS)
 HOST_LDFLAGS:=-L$(STAGING_DIR_HOST)/lib $(if $(IS_PACKAGE_BUILD),-L$(STAGING_DIR_HOSTPKG)/lib -L$(STAGING_DIR)/host/lib)
 
 BUILD_KEY=$(TOPDIR)/key-build
+BUILD_KEY_APK_SEC=$(TOPDIR)/private-key.pem
+BUILD_KEY_APK_PUB=$(TOPDIR)/public-key.pem
 
 FAKEROOT:=$(STAGING_DIR_HOST)/bin/fakeroot