[build][scripts] Build complete ZBI, boot it in QEMU

The primary outputs of the build are now kernel.zbi and zircon.zbi.
The latter is a complete ZBI for standalone booting.
The run-zircon script now uses zircon.zbi by default.
The deprecated -x switch makes it use the old method with direct
kernel loading rather than a trampoline.

The old names zircon.bin and bootdata.bin are made as links, and the
shimmed qemu-zircon.bin is still made. These names are deprecated and
will be removed when all the consumers of the build have been updated.

Test: CQ
Change-Id: I37d8661f8f5766e3947d0a6f71b13178a82af3c4

[build][arm64] Remove remaining uses of the mkbootfs tool

We now use zbi to generate zircon-bootimage.bin,
and use "cat" to prepend the boot shims to the arm64 kernel

Change-Id: Ida69742e77151453f1e76480acbd95fe1ac6f6e1

[arm64] Move platform ID from generated bootdata to the boot shims

With this change, arm64 now uses a common bootdata for all platforms.
We can now update the Fuchsia build system to remove kernel-<board>-bootdata.bin
and stop using per-board bootdatas for "fx boot"

Change-Id: I13213702255a0a05149e78691674ff322c610c59

[kernel][arm64] Remove remaining use of MDI from the arm64 kernel

Kernel drivers are now configured via bootdata passed from the boot shims.

Change-Id: I056b9b2c74a99226d98416b0308480b85460e918

[arm64][boot-shim] Build per-board boot shims

We now build a separate boot shim binary for each board.

Each boot shim now includes a board specific header file
at kernel/target/arm64/board/<board-name>/boot-shim-config.h
which will contain target specific information for the boot shim
to pass to the kernel via the bootdata.

These boot-shim-config.h headers are currently empty, but will be filled
with platform specific information in subsequent CLs.

This CL also fixes gcc dependency support for the boot shim builds.

Change-Id: I0eac0b3ac034d94f9d0730df5740216e6b2f169d

[arm64][boot-shim] Rewrite bulk of boot shim in C

and move boot shim sources to a separate subdirectory

Change-Id: I7c1df33c14809a915adc4bb372503129585925c5

[arm64] Move board directories to kernel/target/arm64/board/

This is in preparation for adding other subdirectories of kernel/target/arm64/
that do not represent boards.

Change-Id: Ic63ac8141ca0963d7cfec02fb853fa2e5de42467