stack/slapos.cfg: Add slapos.extension.shared. This extension creates .shared...

stack/slapos.cfg: Add slapos.extension.shared. This extension creates .shared file in software directory as a mark of software that was built with shared feature.

stack/slapos.cfg

@@ -12,6 +12,7 @@ python = python2.7
 extensions +=
   slapos.rebootstrap
   slapos.extension.strip
+  slapos.extension.shared
 
 show-picked-versions = true
 
@@ -136,6 +137,7 @@ six = 1.10.0
 slapos.cookbook = 1.0.66
 slapos.core = 1.4.9
 slapos.extension.strip = 0.4
+slapos.extension.shared = 1.0
 slapos.libnetworkcache = 0.15
 slapos.rebootstrap = 4.1
 slapos.recipe.build = 0.36