update-kernel: perform word splitting on $modloopfw
The --modloopfw option is supposed to allow multiple extra firmware files to be specified, but the quotes around $modloopfw prevent word splitting. Reported in aports#13408 (closed).
In fact, --modloopfw currently doesn't work at all because the option parsing prepends a space to $modloopfw. This is also the cause of aports#10526 (closed), which happens because mkimg.base.sh passes an empty string to --modloopfw, causing $modloopfw to be " ". The actual warning is Warning: extra firmware " " not found!, contrary to the issue report.