Add android_createEmptyExt4Image()

This patch adds a new function to the emulator to create an
empty EXT4 partition image of a given size in bytes.

+ Basic unit test.

Change-Id: I5b709c5697fe4a57be76b4344413aff3447b616a
diff --git a/Makefile.target b/Makefile.target
index 229190c..ae5e1cd 100644
--- a/Makefile.target
+++ b/Makefile.target
@@ -364,6 +364,9 @@
     emulator-libjpeg \
     emulator-libelff \
     emulator-common \
+    emulator-libext4_utils \
+    emulator-libsparse \
+    emulator-libselinux \
     emulator-zlib \
     $(SDL_STATIC_LIBRARIES)
 
@@ -401,6 +404,9 @@
     emulator64-libjpeg \
     emulator64-libelff \
     emulator64-common \
+    emulator64-libext4_utils \
+    emulator64-libsparse \
+    emulator64-libselinux \
     emulator64-zlib \
     $(SDL_STATIC_LIBRARIES_64)