BUILD: makefile: make all OpenSSL variants use the same settings

BUILD: makefile: make all OpenSSL variants use the same settings

It's getting complicated to configure includes and lib dirs for
OpenSSL API variants such as WolfSSL, because some settings are
common and others are specific but carry a prefix that doesn't
match the USE_* rule scheme.

This patch simplifies everything by considering that all SSL libs
will use SSL_INC, SSL_LIB, SSL_CFLAGS and SSL_LDFLAGS. That's much
more convenient. This works thanks to the settings collector which
explicitly checks the SSL_* settings. When USE_OPENSSL_WOLFSSL is
set, then USE_OPENSSL is implied, so that there's no need to
duplicate maintenance effort.