MIPS: Refactor arch/mips/boot/compressed/Makefile

- use hostprogs-y for the elf2ecoff
- list all *.o file in targets
- renamed obj-y to vmlinuzobjs-y (it was confusing to re-use a kbuild variable)
- fix all uses of if_changed/cmd
- use kbuild rules to beautify output
- update clean-file to clean vmlinuz.* in top-level directory

- simplied logic in arch/mips/Makefile for compressed targets

The net result is a more kbuild conformant Makefile but
readability did not increase.

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/1304/


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