MIPS: Make the debugging of compressed kernel configurable

This patch adds a new DEBUG_ZBOOT option to allow the users to enable it
to debug the compressed kernel support for a new board and this optoin
should be disabled to reduce the kernel image size and speed up the
kernel booting procedure when the compressed kernel support is stable.

Signed-off-by: Wu Zhangjin <wuzhangjin@gmail.com>
To: Ralf Baechle <ralf@linux-mips.org>
Cc: Manuel Lauss <manuel.lauss@googlemail.com>
Cc: linux-mips@linux-mips.org
Patchwork: http://patchwork.linux-mips.org/patch/918/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>