In order to be able to get connected through PPPoA you will the pppoatm.lrp and libatm.lrp packages together with ppp.lrp.

Boot your 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 pppoatm and libatm in the list of packages to be loaded at boot. Check the Bering-uClibc Installation Guide to learn how to do that.

The pppoatm.lrp and libatm.lrp packages are available here.