unified build scheme: a first introduction

The "unified" build scheme revolves around small information files,
build.info, which each describe their own bit of everything that needs
to be built, using a mini-language described in Configurations/README.

The information in build.info file contain references to source files
and final result.  Object files are not mentioned at all, they are
simply from source files.  Because of this, all the *_obj items in
Configurations/*.conf are renamed to *_asm_src and the files listed
in the values are change from object files to their corresponding
source files.  For the sake of the other build schemes, Configure
generates corresponding *_obj entries in %target.

Furthermore, the "unified" build scheme supports having a build
directory tree separate from the source directry tree.

All paths in a build.info file is assumed to be relative to its
location, either within the source tree or within the build tree.

Reviewed-by: Andy Polyakov <appro@openssl.org>