Add a .clang-format file.

Add a .clang-format file.

There's clearly not enough conformance to a single coding style here.
To some extent that can't really change - I don't intend to adopt edk2's
style for the main codebase, but re-formatting the code we borrow from
edk2 would be insane.

This commit adds a .clang-format file, to be used on new files as such:

clang-format --style=file -i foo.c

It can also be used when new free-standing code is added to existing
files, using the clang-format --lines= option.

The starting style in this is pretty close to the Linux kernel style,
with a couple of minor modifications.

Signed-off-by: Peter Jones <pjones@redhat.com>