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).
Oh that!...thats there so that we don't override a user defined `EXTRA_CMDLINE_LINUX`. It's just that in case someone's already exporting `EXTRA_CMDLINE_LINUX` to set the kernel cmdline (since that was the only way to do it previously), we don't want to override that.