The Freescale MXS SoCs have a multi staged boot process which needs
different images composed out of different binaries. The ROM executes
a so called bootstream which contains multiple executables. The first
one is executed in SRAM and the purpose of this binary is to setup
the internal PMIC and the SDRAM. The second image is usually the
bootloader itself. In case of barebox the bootstream is composed
out of the self extracting barebox image (pblx) and the prepare
stage for setting up the SDRAM.

The bootstream image itself is useful for USB boot, but for booting from
SD cards or NAND a BCB header has to be prepended to the image. In case
of SD boot the image has the .mxssd file extension in barebox.

Since the bootstream images are encrypted they are not suitable for
2nd stage execution. For this purpose the 2nd stage images are generated.

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