add function to read single line files

Often small files are used to store the value od a variable. This
adds a function to easily read such a variable.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
cdev: store dos partition type in struct cdev

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Implement bootloader spec support for barebox

The Bootloader Specification describes a way how kernels can
be installed on devices and how they can be started by the bootloader.

The bootloader spec is currently supported by (x86) gummiboot and
by systemd which provides a kernel-install script. With the bootloader
spec it's possible for the Operating system to install a new kernel
without knowing about the bootloader and for the bootloader it's possible
to discover and start Operating Systems on a media without being
configured.

For more details about the spec see:

http://www.freedesktop.org/wiki/Specifications/BootLoaderSpec/

This patch adds barebox support for the spec. It enhances the 'boot'
command so that not only boot script names can be given, but also
devices containing bootloader spec entries. With this it's possible
to call the 'boot' command like: 'boot sd emmc net'. It would then
first look for bootloader spec entries on the (removable) sd card,
then, is nothing is found, on the internal emmc and if still
unsuccessful would call the 'net' bootscript.

The bootloader Spec currently doesn't specify which entry should be
default if multiple entries are found on a single device. Therefore
barebox currently has two extensions of the spec. The $BOOT diretory
can contain a file named 'default'. If present, the content of the
file is treated as a filename under $BOOT/loader/entries/ which is
used as default. Similarly if a file named 'once' is present, the
entry is started once and the file is removed afterwards. This is
useful for testing if a newly installed kernel works before making
it the default.

As on ARM and other Architectures a devicetree has to be specified
for the kernel, the 'devicetree' property is used to specify a
devicetree. Like 'kernel' and 'initrd' this also contains a pth
relative to $BOOT.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
add kernel-install tool for bootloader Spec

This adds a tool for installing kernels according to the bootloader
spec. systemd already has a similar tool, but it is limited to installing
kernels on the currently running system. The barebox kernel-install
tool instead can also be used to install kernels on removable media on a
development host for cross development. It is compiled in two variants,
as 'kernel-install' for the host and as 'kernel-install-target' using
$CROSS_COMPILE.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Merge branch 'pu/bootloader-spec'