boot/uboot: default to kconfig buildsystem for latest version

By default, the builsystem for uboot defaults to 'legacy', while the
default version is very well capable of using the 'kconfig' buildsystem
instead.

Having the 'legacy' buildsystem be the default in that case makes it
quite inconvenient for users: they would expect to be able to use e.g.
uboot-menuconfig et al. with the default uboot version.

Switch to using 'kconfig' when we use the latest version. Keep the
'legacy' as default for everything else.

Also, invert the 'legacy' and 'kconfig' entries in the choice: it is
nicer to have the recent and future-proof entry first.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Tested-by: Petr Vorel <petr.vorel@gmail.com>
Reviewed-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>