ARM: mxs: tools: Use mkimage for BootStream generation

Now that mkimage can generate an BootStream for i.MX23 and i.MX28,
use the mkimage as a default tool to generate the BootStreams instead
of the elftosb tool. This cuts out another obscure dependency.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Otavio Salvador <otavio@ossystems.com.br>
Cc: Stefano Babic <sbabic@denx.de>