Enable Environment Variables in Configuration File

This code change enables the use of Linux Environment
variables in the configuration file. This is very useful
for paths to avoid having to hard-code paths.

Added Environment Variable Checking

For proper use, it will also require a similar patch to
the bundle-chroot-builder project.

See https://github.com/clearlinux/bundle-chroot-builder/pull/12

Signed-off-by: Mark Horn <mark.d.horn@intel.com>

src/builder/builder.go

@@ -115,9 +115,21 @@ func (b *Builder) ReadBuilderConf() {
 
 	for _, h := range fields {
 		r := regexp.MustCompile(h.re)
+		// Look for Environment variables in the config file
+		re := regexp.MustCompile(`\$\{?([[:word:]]+)\}?`)
 		for _, i := range lines {
 			if m := r.FindIndex([]byte(i)); m != nil {
-				*h.dest = i[m[1]:]
+				// We want the variable without the $ or {} for lookup checking
+				matches := re.FindAllStringSubmatch(i[m[1]:], -1)
+				for _, s := range matches {
+					if _, ok := os.LookupEnv(s[1]); !ok {
+						helpers.PrintError(fmt.Errorf("buildconf contains an undefined environment variable: %s", s[1]))
+						os.Exit(1)
+					}
+				}
+
+				// Replace valid Environment Variables
+				*h.dest = os.ExpandEnv(i[m[1]:])
 			}
 		}
 	}