freescale-imx: Add helper to extract self-extractible binaries

[Thomas:
 - reword the comment of the new macro, and add details about the
   expected arguments, as suggested by Yann E. Morin.
 - rework the logic to move the files in $(@D), as it was simply not
   working in the original patch. $(basename $(1)) was used to get the
   filename without the extension, but it also contains the full
   path. $(basename $(notdir $(1))) is needed.]

Signed-off-by: Jérôme Pouiller <jezz@sysmic.org>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>