[build][arm64] Unified build for all arm64 targets

Zircon now has a single unified arm64 build.
Instead of multiple build outputs for different arm targets,
there is now a single build-arm64/ containing a common
arm64 kernel and userspace.

Within build-arm64/ we also build custom <board>-bootdata.bin
and <board>-kernel-bootdata.bin files for each arm64 board.
These differ only by their kernel MDI and platform ID records.

To build the unified arm64 build, type "make arm64" or run the
scripts/build-zircon-arm64 script.

The following scripts were updated to use the new unified arm64 build:

scripts/build-bootable-odroidc2-sdcard.sh
scripts/fastboot-flash
scripts/flash-hikey
scripts/flash-vim
scripts/flash-vim2
scripts/run-zircon-arm64

Change-Id: Ie999f3b4adc938ece29af4fa0d6e41f655972189

[build] Remove unnecessary kernel/project/target/ makefiles

Eliminate an unnecessary layer and fold these into the
kernel/project/<project-name>.mk files instead.

Change-Id: Icb9142fb70bfd1304198b4fe5add262a4bc7f49b

[build] Simplify target ("project") names

The only projects supported now are Zircon, so having "zircon-"
at the front of every name is just more typing. There is only
one architecture associated with each platform name, and for
x86-64 there is only one platform. The old -> new mappings are:

zircon-pc-x86-64 -> x86
zircon-qemu-arm64 -> arm64
zircon-gauss-arm64 -> gauss
zircon-hikey960-arm64 -> hikey960
zircon-odroidc2-arm64 -> odroidc2

The old project names continue to work as aliases during
a transition period but they are deprecated.

Since all projects are Zircon (nee Magenta), the -m switch to
buildall is removed and the build-all-zircon script is deprecated.

Change-Id: Icf88e93b460df053be251c88d9dffd8c6da82518