Add missing apps/openjdk8/Makefile

Add missing makefile so that "make image=openjdk8" will work,
without needing to manually do "cd apps/openjdk8; ./GET" first.

Because openjdk8 compilation is so ridiculously slow, this
Makefile doesn't re-fetch and re-compile it every time.
You can "make clean" to remove the build output,
do the fetch and compile again.

Note that openjdk8 is notoriously difficult to compile
and requires many prerequisites.  Prefer openjdk8-fedora to use
pre-compiled binaries (you don't need to be running Fedora yourself!).

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>