LoongArch: Using generic scripts/install.sh in `make install`

Use the generic script/install.sh to perform the make install operation.
This will automatically generate the initrd file and modify the grub.cfg
without manual intervention (The previous kernel image, config file and
System.map will also be generated), similar to other architectures.

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