mark Linux-specific configuration options

PLATFORM_LINUX is used as a dependency for applets or features
which require Linux-specific interfaces.

Signed-off-by: Jeremie Koenig <jk@jk.fr.eu.org>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

shell/cttyhack.c

@@ -13,6 +13,7 @@
 //config:config CTTYHACK
 //config:	bool "cttyhack"
 //config:	default y
+//config:	depends on PLATFORM_LINUX
 //config:	help
 //config:	  One common problem reported on the mailing list is "can't access tty;
 //config:	  job control turned off" error message which typically appears when