OPTIM: log: use lf_buildctx's buffer instead of temporary stack buffers

OPTIM: log: use lf_buildctx's buffer instead of temporary stack buffers

Now that lf_buildctx isn't pushed on the stack anymore, let's take this
opportunity to store a small buffer of 256 bytes within it, and then use
this buffer as general purpose buffer to build fixed-length strings that
are then printed using lf_{raw}text() function. By doing so we stop
relying on temporary stack buffers.