[U-Boot] SPL Linux Boot V13

Rebased patchset to add Linux booting from SPL. As discussed in the ML, this patchset does not make SPL common to all SOCs to avoid breaking some architecture - this job will be done later with a different patchset.

For this reason, a couple of patches moving files in the common/ directory are dropped - the other patches are not changed from V12. (exception due only to rebase on current u-boot-ti)