Backport ext4 support from master [DO NOT MERGE]

ext4 support is needed by crespo.

Change-Id: I604cb9ada526ce8ba6b3648171ac1d614a5519a9
diff --git a/core/config.mk b/core/config.mk
index 81842be..897af83 100644
--- a/core/config.mk
+++ b/core/config.mk
@@ -206,6 +206,8 @@
 APICHECK := $(HOST_OUT_EXECUTABLES)/apicheck$(HOST_EXECUTABLE_SUFFIX)
 FS_GET_STATS := $(HOST_OUT_EXECUTABLES)/fs_get_stats$(HOST_EXECUTABLE_SUFFIX)
 MKEXT2IMG := $(HOST_OUT_EXECUTABLES)/genext2fs$(HOST_EXECUTABLE_SUFFIX)
+MAKE_EXT4FS := $(HOST_OUT_EXECUTABLES)/make_ext4fs$(HOST_EXECUTABLE_SUFFIX)
+MKEXT2USERIMG := $(HOST_OUT_EXECUTABLES)/mkuserimg.sh
 MKEXT2BOOTIMG := external/genext2fs/mkbootimg_ext2.sh
 MKTARBALL := build/tools/mktarball.sh
 TUNE2FS := tune2fs