MFC r337559: Makefile.inc1: Add libl to -legacy as well

libl is needed for config(8), which is a bootstrap-tool. It is possible to
build a system WITHOUT_TOOLCHAIN to exclude lex and thus, libl. We still
need to support building from this kind of host, though.

While here, group the config(8) dependencies together and add a small
explanation. These can likely both be scoped more clearly, but this will
need some further investigation.