[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.
Also in general we don't build documentations, so remove the documentation
build from the makefile to make builds work.

Signed-off-by: Kalle Jokiniemi <kalle.jokiniemi@jolla.com>