Provide config helpers for environment variables

Add GetenvWithDefault, IsEnvTrue, and IsEnvFalse helpers to Config.

Test: m -j checkbuild
Change-Id: Id855e5349115eb2a18b2e73cfd1bd84b5874ff93
diff --git a/android/config.go b/android/config.go
index fdc7375..4a59bad 100644
--- a/android/config.go
+++ b/android/config.go
@@ -291,6 +291,24 @@
 	return val
 }
 
+func (c *config) GetenvWithDefault(key string, defaultValue string) string {
+	ret := c.Getenv(key)
+	if ret == "" {
+		return defaultValue
+	}
+	return ret
+}
+
+func (c *config) IsEnvTrue(key string) bool {
+	value := c.Getenv(key)
+	return value == "1" || value == "y" || value == "yes" || value == "on" || value == "true"
+}
+
+func (c *config) IsEnvFalse(key string) bool {
+	value := c.Getenv(key)
+	return value == "0" || value == "n" || value == "no" || value == "off" || value == "false"
+}
+
 func (c *config) EnvDeps() map[string]string {
 	c.envLock.Lock()
 	c.envFrozen = true