LoongArch: Add a default install.sh

As specified in scripts/install.sh, the priority order is as follows
(from highest to lowest):
  ~/bin/installkernel
  /sbin/installkernel
  arch/loongarch/boot/install.sh

Fallback to default install.sh if installkernel is not found.

Signed-off-by: Youling Tang <tangyouling@kylinos.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>