tools: mxsboot: pre-fill buffer with 0xff, not 0

tools: mxsboot: pre-fill buffer with 0xff, not 0

The tool works for me, with imx28 and NAND memory, but the resulting
blocks are reported as bad, both by u-boot and the kernel.

This makes it impossible to erase from Linux (for an upgrade without
console access, for example -- u-boot can "nand scrub" but linux can't).

pre-filling with 0xff creates a proper boot loader image, but no
bad-block marker is there when written to flash.

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Marek Vasut <marex@denx.de>