string: Fix (v)asprintf prototypes ...

Our asprintf and vasprintf have different prototypes than the glibc
functions. This causes trouble when we want to share barebox code
with userspace code. Change the prototypes for (v)asprintf to match
the glibc prototypes. Since the current (v)asprintf are convenient
to use change the existing functions to b(v)asprintf.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

include: move shell prototypes to shell.h ...

We have a shell,h, so move shell specific prototypes there.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

include: move run_command prototype to command.h ...

run_command fits much better into command.h, move it there.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

move file helper functions to separate file ...

We have our file helper functions in several places. Move them
all to lib/libfile.c.
With this we no longer have file helpers in fs/fs.c which contains
the core fs functions and no functions in lib/libbb.c which are
not from busybox.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

add menutree command ...

Creating menus from the shell using the regular 'menu' command is rather
complicated. This adds a 'menutree' command which creates a menu from
a directory structure. In the directory structure each directory corresponds
to a single menu entry. The directory contains the following files:

title -  A file containing the title of the entry as shown in the menu
box -    If present, the entry is a 'bool' entry. The file contains a variable
         name from which the current state of the bool is taken from and saved
         to.
action - if present this file contains a shell script which is executed when
         when the entry is selected.

If neither 'box' or 'action' are present this entry is considered a submenu
containing more entries.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>