boot/uboot: add support for the kconfig infrastructure

With the patchset "kconfig: turnaround into single .config"
[http://lists.denx.de/pipermail/u-boot/2015-February/205490.html

]
U-Boot switched to a single .config file for board configuration. This
allows us to use the kconfig-package infrastructure.

For providing backward compatibility with older U-Boot version a user
choice between the new Kconfig and the legacy build system is
introduced.

[Thomas:
  - make use of the legacy build system the default, to not break
    existing configurations.
  - add some comments in the code checking all the configuration
    options, in order to hopefully make it a bit clearer.]

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>