package: rename <pkg>_CONFIG_FIXUP to <pkg>_CONFIG_SCRIPTS

This commit renames the newly introduced <pkg>_CONFIG_FIXUP variable
to <pkg>_CONFIG_SCRIPTS, for two reasons:

 * <pkg>_CONFIG_SCRIPTS will not only "fixup" the scripts in
   $(STAGING_DIR)/usr/bin, but also remove them from
   $(TARGET_DIR)/usr/bin. So it is not only about doing a "fixup".

 * On the principle, it is strange that the variable carries an
   indication of the action that will take place on those files. It
   should rather be named to say "Here are the <foo>-config scripts",
   and let the package infrastructure decide if it should fix them up,
   remove them, etc.

This commit also updates the documentation accordingly.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Acked-by: "Samuel Martin" <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>