arm64 entry point

[Geoff Levand]

Hi,

There is some discrepancy between what booting.txt says and what our
linker script defines as the kernel entry point.

Here is booting.txt:

  The primary CPU must jump directly to the first instruction of the
  kernel image.

And the linker script:

  ENTRY(stext)

Should we change booting.txt, or the linker script?

-Geoff