Changes

vty: Fix extern var declared inside ifdef

"extern struct host host;" was declared inside "ifdef HAVE_SYSLOG_H",
but it is actually needed unconditionally because it's also used in the
gsmtap log target, which is always built.

Change-Id: I9d769ff007a4eaf613691a0491b0b4e07472df98

Add --disable-syslog-logging configure option

Right now syslog logging support is built in automatically based on
syslog.h being built at compile time. That's not desirable; as with
other dependencies make sure there's an explicit configure option for
it. Leave it enabled by default since most systems come with syslog.h
and it's a feature nice to have by default. It is disabled explicitly
when compiling for arm-none-eabi since the toolchain doesn't have
syslog.h.

Take the chance to completelly skip compiling the logging_syslog.c file
if support for syslog logging is disabled.

Change-Id: Ifc54d60e24547696b3059b6eb75572f8c84874d0