Hello Sandra Trujillo García,
This is normal, the file /etc/network/interfaces is provided in the package init-ifupdown, you could modify the configuration file in this package and rebuild it into the rootfs.
Please modify the file sources/meta-freescale/recipes-core/init-ifupdown/init-ifupdown/qoriq/interfaces in Yocto, for example:
auto fm1-mac9
iface fm1-mac9 inet static
address 192.168.1.3
net mask 255.255.255.0
Then rebuild package init-ifupdown and regenerate the rootfs filesystem.
$bitbake init-ifupdown -c cleansstate
$bitbake init-ifupdown
$bitbake fsl-image-core
Have a great day,
TIC
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