Product-configurable sparse ext images.

Also pass extfs sparse image flag to the release tool.

Change-Id: I79a6a71873634a231aa8990cee22374259601505
diff --git a/core/config.mk b/core/config.mk
index 0dec4be..1282ad6 100644
--- a/core/config.mk
+++ b/core/config.mk
@@ -210,7 +210,7 @@
 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
+MKEXTUSERIMG := $(HOST_OUT_EXECUTABLES)/mkuserimg.sh
 MKEXT2BOOTIMG := external/genext2fs/mkbootimg_ext2.sh
 MKTARBALL := build/tools/mktarball.sh
 TUNE2FS := tune2fs