MIPS: Refactor arch/mips/boot/Makefile

- remove stuff that is not needed
  VMLINUX assignment, all: rule
- use hostprogs-y for the host program
- use kbuild rules for the three targets - to beautify output
- drop clean-files - it is no longer needed
- simplify arch/mips/Makefile when calling targets in boot/Makefile

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
To: linux-mips <linux-mips@linux-mips.org>
To: Wu Zhangjin <wuzhangjin@gmail.com>
Patchwork: https://patchwork.linux-mips.org/patch/1303/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>