qemu: fix qemu-native pkg-config paths

For qemu-native, the pkg-config paths do not
include build host paths.
This is an issue for libsdl for example, where SDL is
used by qemu, but for qemu-native libsdl-native is not
built, but assumed to be provided by the build host.
Because pkg-config do not search for libsdl config files
on the build host sysroot, the configure stage of qemu-native
will fail because it will not find SDL as being installed.
Usually, the isssue is masked by a functional sdl-config that
will be interogated instead of pkg-config. However, on Build
Appliance, sdl-config is deliberately made non-functional,
so the issue manifests itself.

The fix will create an extended PKG_CONFIG_PATH, which does
include the build host sysroot paths for pkg-config.

Fix for [YOCTO #6495].

Signed-off-by: Cristian Iorga <cristian.iorga@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>