configs: nitrogen*: bump u-boot version to 2017.07

Also include new boot scripts to match standard distro uboot commands.

Our previous 6x_bootscript and 6x_upgrade used custom variables and
their naming don't make sense now that we target more than i.MX6 CPU.

Therefore those old scripts are marked as legacy and kept for now but
the goal is to deprecate and remove them later.

Signed-off-by: Gary Bisson <gary.bisson@boundarydevices.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>