Export EXTRA_CMDLINE_LINUX from machine.make

Issue #65:

Pass EXTRA_CMDLINE_LINUX, defined in machine.make of a platform, to
onie-mk-installer.sh (we were only passing it only if it was set in
the environment).
Currently, even if we define EXTRA_CMDLINE_LINUX in machine, it's not passed to `onie-mk-installer.s` as I was not doing it in `images.make`. This fixes that problem.