Add signature-build make target

When compiling with:

make signature-build ARCH=xxx COMP=xxx

After binary has been roduced, it will be run to
get the signature 'stockfish bench' and this
number will be used as Version, so that it
will be easy to track the original sources
from a binary.

No functinal change.
Retire engine Tag

It is somewhat redundant and could make SF
name too long, so use just Version, in case
of a signature build Version will be set to
'sig-xxx' otherwise, if left empty, we fall
back on usual date stamp.

No functional change.