When U-Boot started using SPDX tags we were among the early adopters and
there weren't a lot of other examples to borrow from.  So we picked the
area of the file that usually had a full license text and replaced it
with an appropriate SPDX-License-Identifier: entry.  Since then, the
Linux Kernel has adopted SPDX tags and they place it as the very first
line in a file (except where shebangs are used, then it's second line)
and with slightly different comment styles than us.

In part due to community overlap, in part due to better tag visibility
and in part for other minor reasons, switch over to that style.

This commit changes all instances where we have multiple licenses (in
these cases, dual license) declared in the SPDX-License-Identifier tag.
In this case we change from listing "LICENSE-A LICENSE-B" or "LICENSE-A
or LICENSE-B" or "(LICENSE-A OR LICENSE-B)" to "LICENSE-A OR LICENSE-B"
as per the Linux Kernel style document.  Note that...