[btrfs-progs] Update to version 3.16

Signed-off-by: Kalle Jokiniemi <kalle.jokiniemi@jolla.com>
[make] Fix compilation by increasing optimization to -O2

Building with mer toolchaing the -O1 option fails to build.
So increase optimization to -O2 which does compile.

Signed-off-by: Kalle Jokiniemi <kalle.jokiniemi@jolla.com>
[doc] remove documentation building

Mer is missing asciidoc package needed by btrfs-progs documentation build.
Removing the documentation build from the Makefile to avoid the
dependency and to make builds work.

Signed-off-by: Kalle Jokiniemi <kalle.jokiniemi@jolla.com>
Merge pull request #1 from kjokinie/master

Update btrfs-progs to version 3.16