software: allows to cross compile driver and userspace applications

[trabucayre]

Makefile in software/user and software/kernel are written to do a native build (ie. build on machine where driver will be used). It's not possible to cross-compile those code for embedded boards with a different architecture and kernel version using cross-toolchain (for instance built by buildroot).

• This PR replace :=by ?=for KERNEL_PATH to allows users to provides an alternate linux root path
• ARCH is introduce, with by default current system architecture, but may be overwritten by user
• CROSS_COMPILE is introduce too, if this env var is not set, native compiler is used, otherwise the cross-compiler will be used instead.

[enjoy-digital]

Thanks @trabucayre! This is merged.