[kernel][arm64][boot-shim] Make the shim also a ZBI trampoline

This changes the boot shim's behavior so that if it receives an
"initrd" image from the native boot loader then it will use that
ZBI in preference to anything appended to the shim itself.

If the "initrd" is a complete ZBI with its own ARM64 kernel, then
the shim can be a bare shim (with no kernel or ZBI appended).
Anything appended to the shim is ignored in this case.

Otherwise, the kernel appended to the shim will be used but other
ZBI items appended will be ignored in favor of the "initrd" ZBI.

Bug: ZX-2224 #comment boot-shim supports trampoline mode
Test: CQ and manual booting of complete ZBI via bare shim in qemu
Change-Id: Ied0ac07840fded45b717e09f18870ab222fec126

[qemu][arm64] Read actual number of CPU cores from Linux device tree

That is, "scripts/run-zircon-arm64 -s <cpus>" now actually works.

Change-Id: I24f6f0045efe924e9e5768feb10e7e392bd84de1

[kernel][arm64] Move CPU and memory configuration to bootdata

CPU and memory configuration information is now provided
to the kernel by the boot shims.
Now MDI is only being used to configure the kernel drivers.

Change-Id: I2a4ec95de53dcce7fec47b5a003e280d47b95b21

[arm64] Move Linux device tree parsing from kernel to the boot shim

The arm64 kernel now requires that x0 points to the bootdata on entry.
The boot shim parses the Linux device tree to find the bootdata
rather than the kernel doing this.
The boot shim is also now responsible for finding the kernel command line
and ram size in the device tree and passing this to the kernel via
additional bootdata records.

mexec and the hypervisor already pass bootdata to the kernel via x0,
so no changes were needed there.

Change-Id: I927930508566b5ae8f26acd053de7b4f040ee194