env: make himport_r() selective on variables

Add 2 new arguments to himport_r():

 o "nvars", "vars": number and list of variables to take into account
   (0 means ALL)

NOTE: This patch does not change the current behaviour.

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