dde_rump: fix int casting warnings
dde_rump: implement rumpuser_dprintf()

Though we do not need this function, it is helpful while debugging.
dde_rump: move memcpy() to rump lib

On ARM in one way or another 'string.h' prototypes will be used. Move
the definitions from rump_fs to the rump library because it is needed
by all rump based servers running on ARM.
tool: add makefile for building rumpkernel tools

The rumpkernel tools are used within the Genode OS Framework tool chain.
tool: rump wrapper script
dde_rump: block encryption server using cgd(4)