mxs: Silence elftosb

The elftosb tool is now called with -d switch, which produces debug
output to the stdout. The debug output is completely useless for
regular operation, so silence it.

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