MIPS: Fix build of compressed image

Changes introduced to arch/mips/Makefile for the generic kernel resulted
in build errors when making a compressed image if platform-y has multiple
values, like this:

make[2]: *** No rule to make target `alchemy/'.
make[1]: *** [vmlinuz] Error 2
make[1]: Target `_all' not remade because of errors.
make: *** [sub-make] Error 2
make: Target `_all' not remade because of errors.

Fix this by quoting $(platform-y) as it is passed to the Makefile in
arch/mips/boot/compressed/Makefile

Reported-by: kernelci.org bot <bot@kernelci.org>
Link: https://storage.kernelci.org/next/next-20161017/mips-gpr_defconfig/build.log
Signed-off-by: Matt Redfearn <matt.redfearn@imgtec.com>
Reviewed-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/14405/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/Makefile

@@ -263,7 +263,7 @@ KBUILD_CPPFLAGS += -DDATAOFFSET=$(if $(dataoffset-y),$(dataoffset-y),0)
 
 bootvars-y	= VMLINUX_LOAD_ADDRESS=$(load-y) \
 		  VMLINUX_ENTRY_ADDRESS=$(entry-y) \
-		  PLATFORM=$(platform-y)
+		  PLATFORM="$(platform-y)"
 ifdef CONFIG_32BIT
 bootvars-y	+= ADDR_BITS=32
 endif