ARM: Add URLs for MLO and U-Boot binaries

taken from http://gumstix.org/how-to/70-writing-images-to-flash.html
U-Boot: rework flash image rule to be more flexible

We can now specify arbitrary content and offsets for each.

Change the default block size to 1k.
ARM: verdex: keep notes about flash content
ARM: overo: fix flash and SD content
ARM: overo: fix bootscript

We actually now get our boot menu!
Update translations from Pootle
ARM: overo: use the correct bootloader, with a NetBSD entry point