avb2.0: integrate avb 2.0 into the build system

Integrate libavb into the build system. Introduce CONFIG_LIBAVB
build option.

Signed-off-by: Igor Opaniuk <igor.opaniuk@linaro.org>
diff --git a/lib/Kconfig b/lib/Kconfig
index dd54516..a77bf1c 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -187,6 +187,20 @@
 
 endmenu
 
+menu "Android Verified Boot"
+
+config LIBAVB
+	bool "Android Verified Boot 2.0 support"
+	depends on ANDROID_BOOT_IMAGE
+	default n
+	help
+	  This enables support of Android Verified Boot 2.0 which can be used
+	  to assure the end user of the integrity of the software running on a
+	  device. Introduces such features as boot chain of trust, rollback
+	  protection etc.
+
+endmenu
+
 menu "Hashing Support"
 
 config SHA1