ARM: imx: mx5: Simplify TFTP server layout on MX53 Menlo board

By removing the "boot" directory in the "m53menlo/boot/fitImage" path,
we simplify the TFTP server directory layout a bit. This also requires a
change to the mmcload command as it (mis-)uses the same variable as the
TFTP boot.

Signed-off-by: Olaf Mandel <o.mandel@menlosystems.com>
Signed-off-by: Marek Vasut <marex@denx.de>