nvramtool: uname in NetBSD doesnt take "-o"

see the Netbsd manual:
http://netbsd.gw.com/cgi-bin/man-cgi?uname++NetBSD-current
Error output needs to be redirected.

Change-Id: I1853a0162e14be0ee9d7971466499af6c72b2427
Signed-off-by: Zheng Bao <zheng.bao@amd.com>
Signed-off-by: Zheng Bao <fishbaozi@gmail.com>
Reviewed-on: http://review.coreboot.org/1545
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>
diff --git a/util/nvramtool/Makefile b/util/nvramtool/Makefile
index 6723a48..fe2efa0 100644
--- a/util/nvramtool/Makefile
+++ b/util/nvramtool/Makefile
@@ -42,7 +42,7 @@
 ifeq ($(OS_ARCH), NetBSD)
 LDFLAGS = -l$(shell uname -p)
 endif
-ifeq ($(shell uname -o), Cygwin)
+ifeq ($(shell uname -o 2>/dev/null), Cygwin)
 LDFLAGS = -lioperm
 CFLAGS += -D__GLIBC__
 endif