nvramtool: Allow build under Cygwin

To build under Cygwin, nvramtool depends upon the package ioperm:
http://openwince.sourceforge.net/ioperm/

for sys/io.h and sys/perm.h.  This change causes the nvramtool make to correctly include and link against these headers and libraries.

Change-Id: If6cd9d324de7bc19830e0018844f42761b28ddd3
Signed-off-by: Raymond Danks <ray.danks@se-eng.com>
Reviewed-on: http://review.coreboot.org/940
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

util/nvramtool/Makefile

@@ -42,6 +42,10 @@ endif
 ifeq ($(OS_ARCH), NetBSD)
 LDFLAGS = -l$(shell uname -p)
 endif
+ifeq ($(shell uname -o), Cygwin)
+LDFLAGS = -lioperm
+CFLAGS += -D__GLIBC__
+endif
 
 all: dep $(PROGRAM)