tool: add makefile for building rumpkernel tools

The rumpkernel tools are used within the Genode OS Framework tool chain
for preparing and populating disk images as well as creating cgd(8)
based cryptographic disk devices.

Execute 'tool/tool_chain_rump build' to build the tools and afterwards
'tool/tool_chain_rump install' to install the binaries. The default
install location is _/usr/local/genode-rump_.

The rumpkernel based tools may be used with the 'tool/rump' management
tool.
tool: add script for managing rumpkernel tools

The rumpkernel based tools are intended to be used by executing
'tool/rump'. Since it covers the most common use cases for these
tools, this script is comparatively extensive, hence giving a short
tutorial seems reasonable:

* Format a disk image with Ext2:

To format a disk image with the Ext2 file system, first prepare the
actual image by executing dd:

! dd if=/dev/zero of=/path/to/disk_image bs=1M count=128

Second, use 'tool/rump' to format the disk image:

! rump -f -F ext2fs /path/to/disk_image

Afterwards the just created file system may be populated with the
content of another directory by executing

! rump -F ext2fs -p /path/to/another_dir /path/to/disk_image

The content of the file system image can be listed by executing

! rump -F ext2fs -l /path/to/disk_image

* Create a encrypted disk image:

Creating a cryptographic disk image based on cgd(4) is done by
executing the following command:
! rump -c /path/to/disk_image

This will generate a key that may be used to decrypt the image
later on. Since this command will _only_ generate a key and NOT
initialize the disk image, it is highly advised to prepare the disk
image by using '/dev/urandom' instead of '/dev/zero' (only new blocks
that will be written to the disk image are encrypted). In addition
while generating the key a temporary configuration file will be
created. Although this file has proper permissions, it may leak the
generated key if it is created on persistent storage. To specify a more
secure directory the '-t' option should be used:

! rump -c -t /path/to/secure/directory /path/to/disk_image

Decrypting the disk image requires the key generated in the previous
step:

! rump -c -K <key> /path/to/disk_image

For now this key has to specified as command line argument. This is
an issue if the shell, which is used, is maintaing a history of
executed commands.

For completness sake let us put all examples together by creating a
encrypted Ext2 image that will contain all files of Genode's _demo_
scenario:

! dd if=/dev/urandom of=/tmp/demo.img bs=1M count=16
! $(GENODE_DIR)/tool/rump -c -t /ramfs -F ext2fs /tmp/demo.img > \
!         /ramfs/key # key is printed out to stdout
! $(GENODE_DIR)/tool/rump -c -t /ramfs -F ext2fs -k <key> \
!         -p $(BUILD_DIR)/var/run/demo /tmp/demo.img

To check if the image was populated succesfully, execute the
following:

! $(GENODE_DIR)/tool/rump -c -t /ramfs -F ext2fs -k <key> -l \
!         /tmp/demo.img