%_defconfig: save the path to the defconfig

We have the BR2_DEFCONFIG feature that saves the path to the defconfig
file that was specified on the command line, so that a later
savedefconfig would immediately save to the right location. This wasn't
done for the defconfigs in the configs/ directory, however, to avoid
accidentally overwriting them.

Now we decided that it would be more useful to overwrite the defconfigs
in the configs/ directory after all. To do this, we pass the path to
that defconfig in the environment.

Cc: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Tested-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>