Some boards like to do their SDRAM setup in code rather than in hardcoded
DCD tables. For these boards the bootloader has to be loaded to the internal
SRAM which is often too small for a full featured barebox. This new SRAM
image type target allows to overcome this limitation. The image created
with the new imx-sram-img rule is a regular imx-image with the difference
that the load size only spans the pblx image and not the whole image including
the payload. The pbl image easily fits into SRAM. The strategy is that this
initial code initializes the SDRAM, loads the whole image to SDRAM and jumps
to it.

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