Fix Ext2FilesystemTest unittests on Brillo.

The Ext2FilesystemTest tests use a set of pre-generated filesystems
stored in sample_images.tar.bz2 and attempt to parse those. This patch
installs the used filesystems in the same directory as the Chrome OS
test environment would generate those files, relative to the unittest
program executed: in a gen/ subdirectory.

Also, an empty filesystem that was previously created from the unittests
is now pre-generated.

The update_engine_unittests program now depends on those modules being
installed and those unittests pass.

Bug: 26955860
TEST=/data/nativetest/update_engine_unittests/update_engine_unittests --gtest_filter=Ext2FilesystemTest.*

Change-Id: I40082e328cec89a7ef4f06d3b8125a953b29efad
diff --git a/sample_images/generate_images.sh b/sample_images/generate_images.sh
index 70fc14b..1c2a83c 100755
--- a/sample_images/generate_images.sh
+++ b/sample_images/generate_images.sh
@@ -156,6 +156,8 @@
     default)
       add_files_default "${mntdir}" "${block_size}"
       ;;
+    empty)
+      ;;
   esac
 
   cleanup "${mntdir}"
@@ -176,6 +178,7 @@
   # Add more sample images here.
   generate_image disk_ext2_1k default 16777216 1024
   generate_image disk_ext2_4k default 16777216 4096
+  generate_image disk_ext2_4k_empty empty $((1024 * 4096)) 4096
   generate_image disk_ext2_ue_settings ue_settings 16777216 4096
 
   # Generate the tarball and delete temporary images.