build: allow packages to declare ABI versions to detect necessary package rebuilds

If a package directly depends on another package that recently changed
its ABI version, it will be cleaned up and rebuilt (assuming quilt is
not used).

This helps with packages that have no stable ABI, e.g. libubox, ubus,
etc.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

SVN-Revision: 39720

include/package-defaults.mk

@@ -32,6 +32,7 @@ define Package/Default
   else
     VERSION:=$(PKG_RELEASE)
   endif
+  ABI_VERSION:=
   ifneq ($(PKG_FLAGS),)
     PKGFLAGS:=$(PKG_FLAGS)
   else