mips: decompressor: do not copy source files while building

As commit 7ae4a78d ("ARM: 8969/1: decompressor: simplify libfdt
builds") stated, copying source files during the build time may not
end up with as clean code as expected.

Do similar for mips to clean up the Makefile and .gitignore.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Tested-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>