openssl: INSTALL_PREFIX renamed to DESTDIR, environ ignored

INSTALL_PREFIX has been deprecated in openssl so it was renamed to DESTDIR and uses the
new `make-install-extra` option added here:  slapos!524

/cc @tomo @jm 

/reviewed-on !87

obs/re6st/buildout.cfg.in

@@ -38,5 +38,4 @@ environment +=
 
 [openssl]
 prefix = %(TARGET)s/parts/${:_buildout_section_name_}
-environment +=
-  INSTALL_PREFIX=%(ROOT)s
+make-install-extra = DESTDIR=%(ROOT)s