scripts: mk-am3xxx-spi-image: fix wrong assumptions about SPI images ...

We assumed that there is a special image format for SPI. This is not
the case. The AM33xx can boot either images generated with omap_signGP
or raw images which only have the image size and load address in front
of the image. Whether these images are booted from SPI or another
boot medium doesn't matter. The only special thing about SPI is that
the image is in big endian format.

- renames mk-am3xxx-spi-image.c to mk-omap-image.c as the image format
  is not only supported by AM3xxx but also by the OMAP SoCs
- removes the option to specify the SoC
- introduces -s to build a big endian image
- detects if an image already is an image generated with omap_signGP

So the behaviour is like this:

raw image -> mk-omap-image -> prepend size/address -> image for SD/MMC
raw image -> mk-omap-image -s -> prepend size/address, big endian swap -> image for SPI
CH image -> mk-omap-image -> nothing, input == output
CH image -> mk-omap-image -s -> big endian swap -> image for SPI

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