treewide: revise library packaging

- Annotate versionless libraries (such as libubox, libuci etc.) with a fixed
  ABI_VERSION resembling the source date of the last incompatible change
- Annotate packages shipping versioned library objects with ABI_VERSION
- Stop shipping unversioned library symlinks for packages with ABI_VERSION

Ref: https://openwrt.org/docs/guide-developer/package-policies#shared_libraries
Ref: https://github.com/KanjiMonster/maintainer-tools/blob/master/check-abi-versions.pl
Signed-off-by: Jo-Philipp Wich <jo@mein.io>

package/system/uci/Makefile

@@ -9,7 +9,7 @@
 include $(TOPDIR)/rules.mk
 
 PKG_NAME:=uci
-PKG_RELEASE:=1
+PKG_RELEASE:=2
 
 PKG_SOURCE_URL=$(PROJECT_GIT)/project/uci.git
 PKG_SOURCE_PROTO:=git
@@ -35,6 +35,7 @@ define Package/libuci
   CATEGORY:=Libraries
   TITLE:=C library for the Unified Configuration Interface (UCI)
   DEPENDS:=+libubox
+  ABI_VERSION:=20130104
 endef
 
 define Package/uci