build: allow simple build system customization with local.mk

build: allow simple build system customization with local.mk

I tend to automate most of the boring and repetitive tasks like firmware
flashing, build config reconfiguration etc., so I always end up adding
`include local.mk` as a last line in my Makefile, where local.mk is
usually symlink to some other place, just to not accidentally delete it
during `git clean`.

Carrying this single uncommited modification along in the development
process is quite PITA, because it's causing problems during Git
workflow, while rebasing etc.

I hope, that I'm not alone using similar workflow, so I believe, that
this modification might be useful for others as well.

Signed-off-by: Petr Štetiar <ynezz@true.cz>