env: add check/apply logic to himport_r()

Change hashtable so that a callback function will decide whether a
variable can be overwritten, and possibly apply the changes.

So add a new field to struct hsearch_data:

 o "apply" callback function to check whether a variable can be
    overwritten, and possibly immediately apply the changes;
    when NULL, no check is performed.

And a new argument to himport_r():
 o "do_apply": whether to call the apply callback function

NOTE: This patch does not change the current behavior.

Signed-off-by: Gerlando Falauto <gerlando.falauto@keymile.com>
Reviewed-by: Marek Vasut <marex@denx.de>