package/nftables: do not build docs

Building docs requires dblatex with a working Jade Wrapper (jw) which
may well be missing on the build machine, and we do not build our own.

Since docs are anyway removed from the target, just do not build them.

Tell ./configure that we do not have the required tools:
  - DBLATEX=no to disable PDF generation
  - DOCBOOK2X_MAN=no, DOCBOOK2MAN=no and DB2X_DOCBOOK2MAN=no to disable
    manpage generation

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>