Add a 'backend' parameter that defaults to 'make' for backward
compatibility. Extract the make build logic from _generate_script()
into a dedicated _build_with_make() method so different backends can
have their own build implementations.

Reviewed-by: Ben Copeland <ben.copeland@linaro.org>
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>

Add tuxmake as an alternative build backend, enabled with the
USE_TUXMAKE=1 environment variable.

Refactor _parse_fragments() to return a list of fragment file paths
instead of a count and update _merge_frags() to accept this list.
Add _build_with_tuxmake() method with basic tuxmake invocation.
Only call _merge_frags() for the make backend since tuxmake handles
fragments via --kconfig-add, added in the next commit.

Reviewed-by: Ben Copeland <ben.copeland@linaro.org>
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>

Use tuxmake --kconfig-add for config fragments instead of merging
them manually. Add --output-dir to control where tuxmake places
build artifacts.

Build the dtbs target for architectures that support it.

Handle multiple defconfigs by passing the first as --kconfig and
the rest as --kconfig-add.

Handle ChromeOS cros:// defconfig by writing the fetched config
fragments to a file and passing it as --kconfig-add on top of
defconfig. This lets tuxmake do the merge and olddefconfig instead
of doing it before tuxmake runs.

Reviewed-by: Ben Copeland <ben.copeland@linaro.org>
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>

Add dtbs_check target support for the tuxmake backend. When dtbs_check
is requested, run only the dtbs_check target instead of the normal
kernel, modules and dtbs targets.

Also skip firmware fetch for dtbs_check builds since it is not needed.

Reviewed-by: Ben Copeland <ben.copeland@linaro.org>
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>