Fix the build by adding a dummy boot.img file.

The boot was broken by a recent change to build/core/Makefile.
It requires a boot.img file in the out/target/product/encore
directory. We use a ramdisk.img file.
This adds an empty boot.img file which keeps things happy.
This dummy file isn't included in the final flashable zip.
We still don't do all the signing requested by cm but we
do build without errors.

Change-Id: Id1ee0425e034d2c4b7764fd51355469cfc971467