tool: wrapper script for 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
dde_rump: block encryption server using cgd(4)

The 'rump_cgd' server provides block level encryption for a block
session by employing the 'cgd(4)' device provided by the rumpkernel.

'rump_cgd' uses a Block_session to get access to an existing block
device and provides another Block_session to its clients. Each block
written or read by the client is transperently encrypted or decrypted
by the server.

For now 'rump_cgd' may only _configure_ a 'cgd' device but is unable
to generate a configuration. The used cipher is hardcoded to
_aes-cbc_ with a keysize of 256 bit. Furthermore the server is able to
serve one client only.

To ease the usage, its interface is modelled after the interface of
'cgdconfig(8)'. As implications thereof the key must have the same
format as used by 'cgdconfig'. That means the key is a base 64 encoded
string in which the first 4 bytes denote the actual length of the key
in bits (these 4 bytes are stored in big endian order).

Preparing a raw (e.g. without partition table) encrypted Ext2 disk
image is done by executing 'tool/rump':

! dd if=/dev/urandom of=/path/to/disk_image
! rump -c /path/to/disk_image # key is printed to stdout
! rump -c -k <key> -F ext2fs /path/to/disk_image

To use this disk image the following config snippet can be used:

! <start name="rump_cgd">
! 	<resource name="RAM" quantum="8M" />
! 	<provides><service name="Block"/></provides>
! 	<config action="configure">
! 		<params>
! 			<method>key</method>}
! 			<key>AAABAJhpB2Y2UvVjkFdlP4m44449Pi3A/uW211mkanSulJo8</key>
! 		</params>
! 	</config>
! 	<route>
! 		<service name="Block"> <child name="ahci"/> </service>
! 		<any-service> <parent/> <any-child/> </any-service>
! 	</route>
! </start>

the Block service provided by rump_cgd may be used by a file system
server in return:

! <start name="rump_fs">
! 	<resource name="RAM" quantum="16M"/>
! 	<provides><service name="File_system"/></provides>
! 	<config fs="ext2fs">
! 		<policy label="" root="/" writeable="yes"/>
! 	</config>
! 	<route>
! 		<service name="Block"> <child name="rump_cgd"/> </service>
! 		<any-service> <parent/> <any-child/> </any-service>
! 	</route>
! </start>

Since 'tool/rump' just utilizes the rumpkernel running on the host
system to do its duty there is a script called 'tool/cgdconf' that
extracts the key from a 'cgdconfig(8)' generated configuration file
and also is able to generade such a file from a given key. Thereby
the interoperabilty between 'rump_cgd' and the general rumpkernel
based tools is secured.