rsyslog: make libyaml support optional via RSYSLOG_libyaml

rsyslog: make libyaml support optional via RSYSLOG_libyaml

The upstream rsyslog build links against libyaml-0.so.2 whenever
pkg-config detects yaml-0.1, which made libyaml a hard dependency
of the rsyslog binary without any way to opt out from OpenWrt
config. libyaml is only used for YAML (.yaml/.yml) configuration
files and for loading rate-limiting policies from external files;
RainerScript (.conf) installs do not need it.

Introduce a new RSYSLOG_libyaml Config.in switch, default off, and
gate the +libyaml DEPENDS entry on it. Pass --enable-libyaml or
--disable-libyaml to configure based on the switch.

The upstream configure script in 8.2604.0 does not understand a
--disable-libyaml flag (libyaml was unconditionally autodetected).
Backport upstream commit c5c244861 ("configure: make libyaml
default-on explicit") as 001-configure-make-libyaml-default-on-
explicit.patch, which adds the AC_ARG_ENABLE(libyaml) block. The
patch dissolves cleanly once the package is bumped to 8.2606.0+.
PKG_FIXUP:=autoreconf is added so the configure.ac change flows
into the generated configure script during the SDK build.

Signed-off-by: Alexandru Ardelean <redacted>