makefile: parse and process flags to enable ffbm mode on systemd for LE

Pass a flag to make utility to ensure "systemd.target" kernel
commandline parameter is added when systemd is
used as the init manager on LE platform. This is
done by reading flags sent by EXTRA_OEMAKE in the
lk recipe file.

Change-Id: Ic9330c260fd03b25122c90a1c97bcdec48b14570
1 file changed