[expr] rewrite evaluator to separate package

this moves evaluator to separate package which interfaces with syscfg
via interface. this removes direct dependency on syscfg package.

Author: andrzej-kaczmarek

newt/cli/util.go

@@ -40,6 +40,8 @@ import (
 	"mynewt.apache.org/newt/util"
 )
 
+var GlobalResolver *resolve.Resolver
+
 const TARGET_KEYWORD_ALL string = "all"
 const TARGET_DEFAULT_DIR string = "targets"
 const MFG_DEFAULT_DIR string = "mfgs"