uboot: add support for generating U-Boot boot scripts

uboot: add support for generating U-Boot boot scripts

More and more of our defconfigs need to generate a U-Boot boot
script. It's a simple call to mkimage, but we already have 12
instances of this logic in board/, and there are patch series waiting
in patchwork adding 3 more boards that need this.

So let's add an option in the U-Boot package to generate such a boot
script image easily.

Note that we assume a single script needs to be generated, and the
output file name is boot.scr. The only platform for which it seems to
not be the case are the Boundary Devices platforms: they generate two
boot scripts, 6x_bootscript and 6x_upgrade, but they are anyway
installed inside TARGET_DIR, not BINARIES_DIR.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>