Added build ("make") framework for compiling Java code we want to run in
OSv (in/with the Java payload), as well as JNI, i.e., OSv-specific C code
we want to run from the above Java code.

The Java source files in java/src/ are now all compiled during build
(see java/build.xml for the Ant file doing this compilation) and the
result is one JAR, build/$mode/java/cloudius.jar (/cloudius.jar in bootfs).
We can easily change build.xml to create more than one if we want.

As an example of JNI, I used the not-working-yet balloon feature.
java/src/com/cloudius/balloon/Balloon.java is the class
com.cloudius.balloon.Balloon which is supposed to have one "native"
(implemented in C) function giveup(). This function's implementation is
in java/jni/balloon.c. We get this file to compile to balloon.so (put
in /usr/lib/jni in the bootfs) by adding java/jni/balloon.so to the
"jni" list in build.mak.

If you don't know how write the ugly function signature as seen in
java/jni/balloon.c, you can run "javah com.cloudius.balloon.Balloon"
to build the empty functions needed for implementing the native functions
defined in the above class.