Append arguments to /boot/cmdline.txt

[killerwhile]

It might be interesting in some scenarii to add extra parameters to /boot/cmdline.txt, for instance cgroup_enable=memory cgroup_memory=1 to enable docker on R-pi.

The new feature proposes --cmdlineargs arguments to flash to achieve this exact goal.

[LS80]

This would also be useful for me 👍
Currently I use --file to copy a complete version of cmdline.txt, but then you can't also copy a network-config file because you can only specify --file once.
Perhaps it would also be useful to have a dedicated flag to copy a network-config file.

[killerwhile]

Allowing to have more than one --file is also a good idea! Out of the scope of this PR though.