Make savedefconfig save to a configured file.

Store BR2_DEFCONFIG in .config, and use it to update the original input
defconfig file after updating the configuration.  When a config is
created by using the BR2_DEFCONFIG=... option, this is saved in the
.config file; later runs of savedefconfig will update that same location.
It is also possible to configure this place in the interactive
configuration.

The BR2_DEFCONFIG value itself is not saved into the generated
defconfig, since Kconfig considers it at its default. This is
intentional, to avoid hard-coding an absolute path in the defconfig.
It will anyway be set again when the defconfig is used with the
'make BR2_DEFCONFIG=... defconfig' command.

As a side-effect of this change, the *config options have been moved out
of the BR2_HAVE_DOT_CONFIG condition.  This doesn't make any functional
difference, because the .config is still not read for the *config targets.
However, the defconfig and savedefconfig targets do need to include
.config now, which makes them slightly slower.

[Peter: slightly tweak help text]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Acked-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>