Revert "Remove the MD5 implementation that was only used by toolbox."

This reverts commit 67401638a92abd52e5896183e851b1b097da07ac.

Added TARGET_NEEDS_BIONIC_MD5 to indicate if a device needs it.
In particular, the Galaxy Nexus RIL breaks without this.

Change-Id: I49f1779566dfeb5093970ed0bbda741919318a83
diff --git a/libc/Android.mk b/libc/Android.mk
index c6b8cee..fcf7c67 100644
--- a/libc/Android.mk
+++ b/libc/Android.mk
@@ -63,6 +63,10 @@
     stdio/snprintf.c\
     stdio/sprintf.c \
 
+ifeq ($(TARGET_NEEDS_BIONIC_MD5),true)
+libc_common_src_files += bionic/md5.c
+endif
+
 # Fortify implementations of libc functions.
 libc_common_src_files += \
     bionic/__FD_chk.cpp \