[kernel] Remove kernel/app module

This is no longer used at all.
The kernel/app/tests code doesn't use the "app" infrastructure.
So move it to kernel/tests.

Change-Id: I6cd8ec1bf569fd95940e5ebf4dc5f640b265eb69

[kernel][top] move the entry point and init code to c++

Run it through the reformatter and delete some old code while at it

Change-Id: Id45ca421886c1af6ef64092fec538ed16d11c365

[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

[ulib][magenta] Add mx_version_get vDSO call

mx_status_t mx_version_get(char* ver, uint32_t len)

Probably temporary in form but very useful.
The string returned can be used with "git show".

- Print version in MOTD
- Change dot for '-' from MOTD, it does not show in graphical console

Change-Id: Ic777d6c04ce31daa3846a90ec6ec75c5ac820d96

[magenta] Initial commit