lantiq: remove "init" kernel command line parameter from bootargs

/etc/preinit has been the default init-script for a very long time (at
least since Linux 2.6.30 in OpenWrt). Remove the kernel command line
"init" parameter to get rid of duplicate and inconsistent definitions
of this parameter (some boards, like FRITZ3370 for example did not use
it at all, while it's just copy and paste on others).

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>