Move more build configuration to openssl.config

This patch makes openssl.config the only place where common
and architecture-specific sources and compiler flags are listed.

Its content is processed by import_openssl.sh to generate new
build config files (Crypto-config.mk, Ssl-config.mk, Apps-config.mk)
which are themselves included by simplified Crypto.mk, Ssl.mk, Apps.mk.

+ Add a new script (check-all-builds.sh) that can rebuild six
  different variants of openssl in one go. This is useful to quickly
  check that a change didn't break a specific build, e.g. when
  adding  new patch or upgrading the OpenSSL sources.

  See './check-all-builds.sh --help' for more info.

  Note: Clang-based builds are currently broken, so only GCC-based
        ones are activated at the moment.

Change-Id: If08c204e4dc9b081ce676bc7984d039670e115b0