Use builtin eCos libm functions when not provided by external library

For eCos-based firmwares, we don't want to use libm which comes from the toolchain, since eCos brings a libm itself and we fear collisions. Thus, for the first beta release, we rolled these functions our own.

Instead, the implementation should be adapted to fall back to the builtin libm functions provided by eCos when no external libraray is linked implementing those functions.