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/Makefile b/lib/Makefile
index 427d359..5f583ae 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -56,6 +56,7 @@
 obj-$(CONFIG_$(SPL_)GZIP) += gunzip.o
 obj-$(CONFIG_$(SPL_)LZO) += lzo/
 
+obj-$(CONFIG_LIBAVB) += libavb/
 
 obj-$(CONFIG_$(SPL_TPL_)SAVEENV) += qsort.o
 obj-$(CONFIG_$(SPL_TPL_)OF_LIBFDT) += libfdt/