MIPS: Fix vmlinuz build when ZBOOT is selected

vmlinuz is not built by default for platforms using
COMPRESSION_FNAME (e.g. Malta) due to an erroneous
check on ZBOOT
Signed-off-by: Daniel Sabogal <dsabogalcc@gmail.com>
Reviewed-by: James Hogan <jhogan@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/18466/Signed-off-by: James Hogan <jhogan@kernel.org>

arch/mips/Makefile

@@ -228,7 +228,7 @@ libs-y				+= arch/mips/fw/lib/
 #
 # Kernel compression
 #
-ifdef SYS_SUPPORTS_ZBOOT
+ifdef CONFIG_SYS_SUPPORTS_ZBOOT
 COMPRESSION_FNAME		= vmlinuz
 else
 COMPRESSION_FNAME		= vmlinux