MIPS: enable both vmlinux.gz.itb and vmlinuz for generic

vmlinux.gz.itb should be appended to all-$(CONFIG_MIPS_GENERIC)
instead of replacing. Otherwise, no vmlinuz will be built.

Signed-off-by: YunQiang Su <yunqiang.su@cipunited.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>