tools: use pkg-config when available to get SSL flags

tools: use pkg-config when available to get SSL flags

Instead of hardcoding -lssl -lcrypto as the flags needed to build
mkimage with FIT signature enabled, use pkg-config when
available. This allows to properly support cases where static linking
is used, which requires linking with -lz, since OpenSSL uses zlib
internally.

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

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>