apk: add package

apk: add package

APK (Alpine Package Keeper) is the package manager of Alpine Linux and
has multiple advantages over OPKG. While Alpine uses APK version 2, this
commit adds version 3 with a heavily optimised database structure and
additional feature making it suitable for OpenWrt.

This commit will be followed by many more to add APK build capabilities
to the OpenWrt build system, firstly enabling side by side builds of APK
and OPKG packages, later replacing OPKG entirely.

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