Add release target to Makefile

Add simple helper target to automate basic release steps:
* version bump
* prepare release commit
* git commit, tag and sign

For library projects:
* update debian/changelog from TODO-RELEASE
* cleanup TODO-RELEASE

For non-library projects:
* update debian/changelog from git log

Note: it requires bumpversion package to be installed, debian/control is
adjusted accordingly. The helper itself is installed to facilitate reuse
by other libraries.

N. B: you still have to manually adjust LIBVERSION in previous commit -
see TODO-RELEASE header for details.

Use it as follows:
make REL=minor release

The REL parameter defines which component of the version [1] to bump and
can be any of { major, minor, patch }.

[1] http://semver.org/

Change-Id: I790ceb958195b9f6cbabfe8c977dc30e2bd7414b
Related: OS#1861
diff --git a/Makefile.am b/Makefile.am
index b8de3ca..8b35856 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -7,13 +7,20 @@
 pkgconfig_DATA = libosmocore.pc libosmocodec.pc libosmovty.pc libosmogsm.pc \
 		 libosmogb.pc libosmoctrl.pc libosmocoding.pc
 
+@RELMAKE@
+
+relengdir = $(includedir)
+releng_DATA = osmo-release.mk
+
+osmo-release.mk: git-version-gen
+
 BUILT_SOURCES = $(top_srcdir)/.version
 $(top_srcdir)/.version:
 	echo $(VERSION) > $@-t && mv $@-t $@
 dist-hook:
 	echo $(VERSION) > $(distdir)/.tarball-version
 
-EXTRA_DIST = git-version-gen .version README.md
+EXTRA_DIST = git-version-gen .version README.md osmo-release.mk
 
 if HAVE_DOXYGEN