Merge branch 'builtin-clean'

Support in-tree builtin module building.

These commits add support for compiling the SPL module as a built-in
kernel module by copying the module code into the kernel source tree.
Here's the procedure:

  - Create your kernel configuration (`.config` file) as usual. This
    has to be done first so that SPL's configure script is able to
    detect kernel features correctly.
  - Run `make prepare scripts` inside the kernel source tree.
  - Run `./configure --enable-linux-builtin --with-linux=/usr/src/linux-...`
    inside the SPL directory.
  - Run `./copy-builtin /usr/src/linux-...` inside the SPL directory.
  - In the kernel source tree, enable the `CONFIG_SPL` option
    (e.g. using `make menuconfig`).
  - Build the kernel as usual.

SPL module parameters can be set at boot time using the following syntax
on the kernel command line: `spl.parameter_name=parameter_value`.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue zfsonlinux/zfs#851