pxe: add AArch64 image support

pxe: add AArch64 image support

The sysboot and pxe commands currently support either U-Boot formats or
raw zImages. Add support for the AArch64 Linux port's native image format
too.

As with zImage support, there is no auto-detection of the native image
format. Rather, if the image is auto-detected as a U-Boot format, U-Boot
will try to interpret it as such. Otherwise, U-Boot will fall back to a
raw/native image format, if one is enabled.

My belief is that CONFIG_CMD_BOOTZ won't ever be enabled for any AArch64
port, hence there's never a need to differentiate between CONFIG_CMD_
_BOOTI and _BOOTZ at run-time; compile-time will do. Even if this isn't
true, we want to prefer _BOOTI over _BOOTZ when defined, since _BOOTI is
definitely the native format for AArch64.

Change-Id: I83c5cc7566032afd72516de46f4e5eb7a780284a
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>