Add in-system barebox update infrastructure ...

Currently in-system update means to write an arbitrary file to
an arbitrary device. There is no sanity check if the flashed image
is of the right type or will fit onto the device. Furthermore some
SoCs need a special preparation step for their images before
flashing them.

This adds a barebox in-system update infrastructure. Boards can
register update handlers which know how to make the board bootable.
The available handlers can be listed to be able to select one,
different force levels give the user the chance to know it better.

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