boot: arm-trusted-firmware-microchipsw: fix compilation against LibreSSL

boot: arm-trusted-firmware-microchipsw: fix compilation against LibreSSL

LibreSSL 3.9+ has dropped support for X509V3 extension API so cert_create
tool does not compile against it at all.

This was hidden by the fact that it was compiling against OpenSSL on my
host which still has that API, however we do not ship libssl-dev in the
Buildbot containers so compiling against distro OpenSSL is not possible.

So, after a long time trying to find any docs on that API I resorted to
LLM(Gemini 3 Pro) to get it to compile.

Our libcrypto is linked against pthread so we must pass -lpthread as well
for cert_tool.

Fixes: 5205c0c42607 ("microchipsw: lan969x: add Microchip EV23X71A")
Signed-off-by: Robert Marko <robert.marko@sartura.hr>
(cherry picked from commit 410277ca12d811daa8040edf75dfd87a2d9dd5f8)