linux: improve the 'make linux-*' special commands

This patch improves two things:

 * It makes sure that the linux-menuconfig and al. commands can be
   executed even if the user hasn't executed 'make' before. Until now,
   the commands were depending on the .configured stamp, which is not
   sufficient since the linux package has been converted to the
   GENTARGETS infrastructure. Instead, depend on the linux-configure
   phony target.

 * It makes sure that those commands are not available when the Linux
   package is not selected, just like we do for Barebox.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>