kexec-tools: mips: Concatenate --reuse-cmdline and --append

Use concat_cmdline() to concatenate the --append string and
the --reuse-cmdline string, otherwise only one of the two
options is valid.

This is similar with commit 8b42c99a ("Fix --reuse-cmdline
so it is usable.").

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Simon Horman <horms@verge.net.au>