Boot a Bering-uClibc floppy image. Once the LEAF menu appears get access to the linux shell by (q)uitting the menu. Edit the lrpkg.cfg (pre Bering-uClibc-2.2.0) or leaf.cfg (Bering-uClibc-2.2.0 onwards) file and replace the dhcpcd entry by ppp in the list of packages to be loaded at boot. Check the Bering-uClibc Installation Guide to learn how to do that.