tools: kwboot: use pkg-config to get -ltinfo

Instead of hardcoding -ltinfo as the flags needed to build
kwboot, use pkg-config when available.

We gracefully fallback on the previous behavior of hardcoding -ltinfo
if pkg-config is not available or fails with an error.

Reviewed-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Heiko Thiery <heiko.thiery@gmail.com>