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>
diff --git a/meta-openbmc-bsp/meta-nuvoton/conf/machine/evb-npcm750.conf b/meta-openbmc-bsp/meta-nuvoton/conf/machine/evb-npcm750.conf
index e06e93a..38c02ed 100644
--- a/meta-openbmc-bsp/meta-nuvoton/conf/machine/evb-npcm750.conf
+++ b/meta-openbmc-bsp/meta-nuvoton/conf/machine/evb-npcm750.conf
@@ -7,3 +7,7 @@
 
 require conf/machine/include/npcm7xx.inc
 require conf/machine/include/obmc-bsp-common.inc
+
+# Don't generate MTD flash images until we're able to include the primary
+# bootloader and the Linux MTD driver is ready.
+IMAGE_FSTYPES = "cpio.${INITRAMFS_CTYPE}.u-boot"