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>

mount: support filesystem options passed via -o ...

Similar to mount(8) the barebox command mount now supports passing a string
to the file system driver via -o.

This is used in the next commit to let the user specify port numbers for
nfs mounts.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

introduce common bootstrap code ...

This will allow to have a generic code to create different bootstrap

As example
Barebox as TI Xloader
Barebox as AT91 Bootstrap

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>