systemd-serialgetty: change to handle SERIAL_CONSOLES

systemd-serialgetty: change to handle SERIAL_CONSOLES

SERIAL_CONSOLES is now set from SERIAL_CONSOLE if not already set.

This is a little bit gnarly because we have to create a separate service
file for each different baud rate; assume that the first baud rate is
the default, thus preserving the previous behaviour in the event there
is only one baud rate in use.

This change also installs the service file before modifying it in place,
allowing do_install to re-execute properly; additionally the service
file now has the correct permissions (i.e. no execute bit set).

(From OE-Core rev: cd89a71d0c3dce14d59134667bc47fa8210cfa7a)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>