[U-Boot] mkimage and XIP vs entry point

[Ken MacLeod]

Re. XIP, we recently did some boot time optimizations and found that
if we loaded the uncompressed kernel-only uImage to RAM (diskboot)
with the load address in the uImage the same as the load address given
to diskboot and the execute addres at +64 then we shaved off 0.15
seconds by letting the kernel do its own relocation instead of U-Boot
relocating the whole image.  (PPC8247 at 66MHz bus, 2.9MB kernel)

Just to be clear, we're *not* using an XIP kernel, we're just letting
it trigger the XIP start condition in cmd_bootm and letting the kernel
do its own relocation magic.  Using an XIP kernel at the RAM-loaded
location would have saved the other 0.10 seconds but it didn't seem
worth the effort to update and integrate the XIP patch.

  -- Ken