target: support generic getty with sysvinit inittabs

In sysvinit inittab the "id" field (first field) must be no longer
than 4 bytes, and is not used by init to determine the output
terminal. Therefore, we adjust the strategy used to modify the inittab
file according to the getty configuration.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>