Hi mbertrand
As gerald_clark pointed out, remove the usb-serial-3.0.3-tinycore extension, install the usb-serial-3.0.21-tinycore
and reboot. The driver is in there, no need to compile your own, it's under:
/usr/local/lib/modules/3.0.21-tinycore/kernel/drivers/usb/serial/ftdi_sio.ko.gz
The strange thing is, when I entered ftdi in the provides field in AppBrowser, none of the usb-serial extensions
showed up. This is under TC4.1. Can someone running a later version of TC confirm this behavior?