Fix non-Linux builds

Various tools are Linuxish and should thus only attempted to build on
Linux only. Some features are also Linux-only.

Also, libresolv is used on all GNU platforms, notably GNU/Hurd and
GNU/kfreeBSD.

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

console-tools/loadfont.c

@@ -12,6 +12,7 @@
 //config:config LOADFONT
 //config:	bool "loadfont (5.2 kb)"
 //config:	default y
+//config:	select PLATFORM_LINUX
 //config:	help
 //config:	This program loads a console font from standard input.
 //config:

console-tools/openvt.c

@@ -10,6 +10,7 @@
 //config:config OPENVT
 //config:	bool "openvt (7.2 kb)"
 //config:	default y
+//config:	select PLATFORM_LINUX
 //config:	help
 //config:	This program is used to start a command on an unused
 //config:	virtual terminal.