mips: Add '--reuse-cmdline' optional parameter support

This patch adds an option "--reuse-cmdline" for people that are lazy
in typing --append="$(cat /proc/cmdline)", which will directly use the
command line of the currently running system.

Signed-off-by: Youling Tang <tangyouling@loongson.cn>
Signed-off-by: Simon Horman <horms@verge.net.au>