tiff: Add packageconfig knob for webp

tiff-native otherwise falsely detects webp if its installed on build
host. This ensures deterministic behavior regardless of host.

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 718c44f2)
Signed-off-by: Steve Sakoman <steve@sakoman.com>