gdb: get rid of host-texinfo dependency

gdb: get rid of host-texinfo dependency

The host-texinfo dependency in gdb/host-gdb was added because gdb insisted
in building info documentation in recent versions, and we want to avoid
'makeinfo' to be present on the build system.

However, there is another solution that does not require actually building
host-texinfo: instruct the makefiles to use a dummy makeinfo command
('true').

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>