ARM: i.MX27 pcm038: switch to multi image ...

This also converts the Phytec phyCORE i.MX27 aka pcm038 to use
image compression. The image will be named

barebox-phytec-phycore-imx27.img

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

ARM: i.MX: Add multi images support Makefile ...

This adds images/Makefile.imx which will contain the i.MX
specific image generation snippets.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>