builddeb: Try to determine distribution

lsb_release command is a good choice to determine the distribution
name for the changelog file in the generated Debian packages [1].
Its installation is no precondition.
In Debian it is still not essential or build-essential.

Ben gave some helpful informations and detailed explanations in [2].
There he also suggested to have an option to explicitly set the
distribution name (see $KDEB_CHANGELOG_DIST variable).

Embedded the improvement as suggested by Thorsten (see [3]):

"This is suboptimal: if KDEB_CHANGELOG_DIST is defined,
lsb_release is not necessary. The following snippet
also omits using its output if it fails but still
produces any:"

Dealing with this issue I learned about "The Colon in the Shell." and
possible pitfalls in this area (see [4,5]). Furthermore, refreshed my
knowledge about redirecting outputs with the echo command (see [5]).
Special thanks to Thorsten, I enjoyed the IRC session with you.

Cooked together the snippets of Ben and Thorsten (see [2,3]).
Tested against Linux v3.19-rc2.

Thanks goes to Alexander, Ben, maximilian and Thorsten for the very
vital help.

[1] https://lkml.org/lkml/2012/4/23/516
[2] http://marc.info/?l=linux-kbuild&m=142022188322321&w=2
[3] http://marc.info/?l=linux-kbuild&m=142023476825460&w=2
[4] http://blog.brlink.eu/index.html#i70
[5] https://www.mirbsd.org/permalinks/wlog-10_e20141209-tg.htm
[6] http://stackoverflow.com/questions/23489934/echo-2-some-text-what-does-it-mean-in-shell-scripting

CC: Alexander Wirt <formorer@debian.org>
Suggested-by: Ben Hutchings <ben@decadent.org.uk>
Suggested-by: Thorsten Glaser <tg@mirbsd.org>
Reviewed-by: Ben Hutchings <ben@decadent.org.uk>
Acked-by: maximilian attems <max@stro.at>
[ dileks: Reviewed his suggested diff in RFC v4 ]
Reviewed-by: Thorsten Glaser <tg@debian.org>
Signed-off-by: Sedat Dilek <sedat.dilek@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>

scripts/package/builddeb

@@ -217,9 +217,20 @@ else
 fi
 maintainer="$name <$email>"
 
+# Try to determine distribution
+if [ -n "$KDEB_CHANGELOG_DIST" ]; then
+        distribution=$KDEB_CHANGELOG_DIST
+elif distribution=$(lsb_release -cs 2>/dev/null) && [ -n "$distribution" ]; then
+        : # nothing to do in this case
+else
+        distribution="unstable"
+        echo >&2 "Using default distribution of 'unstable' in the changelog"
+        echo >&2 "Install lsb-release or set \$KDEB_CHANGELOG_DIST explicitly"
+fi
+
 # Generate a simple changelog template
 cat <<EOF > debian/changelog
-linux-upstream ($packageversion) unstable; urgency=low
+linux-upstream ($packageversion) $distribution; urgency=low
 
   * Custom built Linux kernel.