gdb: prevent building the documentation

Force gdb to not build the documentation. This way we avoid depending on
host-texinfo. This is a temporary fix until upstream accepts a proposed
--disable-docs configure option.

Since the documentation will not be build at all, we can remove the
parts related to host-texinfo and MAKEINFO in the gdb.mk file.

Fixes:

  http://autobuild.buildroot.net/results/dd5/dd50ed99abb2c8495def826866b184030953f90e/



Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Tested-by: Brendan Heading <brendanheading@gmail.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>