Added local directory as source of kernel code

Add the option to use a local directory as the source for
building the Linux kernel, which can be useful during
kernel development.

Signed-off-by: Rafal Fabich <rafal.fabich@gmail.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Tested-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>