[boot-shim] Move debug UART code into board specific source files

Also removed VIM2 support, since it is no longer using the boot shim.

TEST: manual testing on qemu and hikey960, with DEBUG_UART on and off.
Change-Id: Ia25251fb7a857bd7d11dcbf9b546e339ef79f62e

[build][boot-shim] Check that boot shim code is purely position-independent

The script that generates KASLR fixups for the kernel easily doubles
as a checker that an image like a boot shim has no such fixups.

Test: CQ & manually tested build fails with global fixup inserted in shim code
Change-Id: I5186dc6a56d82472d8d7973d0f8ed95a0fac1746

[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

[ulib][zbi] Rename zbi to libzbi

This is to avoid the naming collision with the zbi tool, so that the
ulib can be exported for use by upper layers.

Test: Built Zircon and Garnet, then ran using "fx run -k".
Change-Id: Ib3f495f908791f6808fa4a399d087f24ec501a88

[kernel][arm64][boot-shim] Use zbi library

Bug: ZX-2224 #comment boot-shim cleanup
Test: CQ
Change-Id: I88f6e8bed3c5259a691bb8eeb04ff467df6f5a47

[kernel][arm64][boot-shim] Compile shim code with -mstrict-align

The boot shim code runs with hardware alignment checking enabled,
unlike the kernel proper.

TC-36

Change-Id: I17cab766b545c49ab91c3f24be672d54f2ab7dc2

[arm64][boot-shim] Clean up shim code and link, pad shims

Clean up the shim code a little bit. Keep the shim images
padded out to the kernel alignment, so they can be directly
concatenated with a BOOTDATA image without knowing the alignment.

Change-Id: I603e7345ae82a06844fd7ca5015ceaf06509e7ce

[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

[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