net/dhcp6: Fix dependencies and make parallel build reliable

[leres]

Summary

Proposed commit message:

Fix Makefile dependency so that cfparse.c and y.tab.h exist
before cftoken.c is generated or compiled.

PR: 218859
Submitted by:
Reported by:
Reviewed by: ? (mentor)
Approved by: maintainer timeout (68 weeks), ? (mentor)
Obtained from:
MFC after:
MFH:
Relnotes:
Security:
Sponsored by:
Differential Revision: ?

[ler]

LGTM. Shipit.