meta-nuvoton: evb-npcm750: disable MTD image generation

The OpenBMC build system doesn't currently generate a flash image usable
by NPCM7XX BMCs, and the OpenBMC Nuvoton kernel doesn't have an MTD
SPI-NOR driver.

The MTD image generation build step is failing for the NPCM750 eval
board, so disable it and instead generate a rootfs cpio suitable for
loading from u-boot.

Tested: Built for evb-npcm750 and was able to netboot the board

Change-Id: I8b6275979d78b7d5657e77d25bd74f933c740ea8
Signed-off-by: Benjamin Fair <benjaminfair@google.com>