Hello Nouchi,
g_serial.ko was in /lib/modules/3.0.15-ts-armv7l/kernel/drivers/usb/gadget folder when i tried.
As you said that it was working on your vybrid design, i've re-tried with differents config kernel, and it seems to work, i don't know why. Maybe the OTG_SUPPORT, that is now removed, was in conflict with g_serial...
modprobe g_serial now return :
gs_bind: controller 'fsl-usb2-udc' not recognized
g_serial gadget: Gadget Serial v2.4
g_serial gadget: g_serial ready
fsl-usb2-udc: bind to driver g_serial
For information, do you know what's the meaning of : gs_bind: controller 'fsl-usb2-udc' not recognized ?