kconfig: Use HOST_OS environment variable in Makefile Since the root Makefile already provides a HOST_OS environment variable that is exported by using uname, make use of it in the Kconfig Makefile. Change-Id: I13655a5295bbcd9f2fdfa8b6309634c1ab143f70 Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>