[kernel] Port debuglog to C++.

TEST=Manual

Change-Id: I8d9c5a4c286c26179ae241db34304e3f4fb613e8

[kernel][crashlog] move away from printf callback

instead of piggybacking on the printf messages, we build a crashlog
struct with the relevant fields that we then serialize to string

DX-230 # step 1

TESTED=manual run of `k crash` on device (arm64 and x64) and manual inspection of logs upon reboot

Change-Id: I6019c85cbd0d71084d543667850dea30ac06ea93

[build] flatten the build

Previously we treated kernel/, system/, and third_party/ as
overlays on a shared namespace. This required the concept
of "canonical" module names, and a lot of complexity to ensure
that things didn't collide and the build worked.

This change gets rid of that, no longer passes -I to make,
so that include directives from our *.mk files do not magically
wildcard across various paths, etc.

The most user-visible change is that everywhere where a module
name is specified (MODULE_DEPS, MODULE_LIBS, etc), full module
names like kernel/lib/io or system/ulib/mxio must be used instead
of previously-allowed "short" names like lib/io and ulib/mxio.

The build output still has a similar shape, but the first segment
of the module path (kernel/, system/, or third_party/) is no
longer elided under $(BUILDDIR)

Change-Id: I525aba1da1c86eb7a86007bddc669f7eeebfedd5

[kernel][version] remove git-version.h and flip all kernel uses to the existing lib/version implementation

lib/version builds config-buildid.h which already has the git revision in it.

Change-Id: I0542ff5123e7900fa56b34a3b10141078280273e

[magenta] Initial commit