stand: tftp.c doesn't need bootinfo.h

tftp.c includes bootinfo.h, but doesn't need it. Remove it, and remove
the -Istand/common from CFALGS since that's the only reason we had it.

Sponsored by: Netflix
Reviewed by: tsoome
Differential Revision: https://reviews.freebsd.org/D35928

(cherry picked from commit 60cb4f9a8e258e917634d23fb1d589eef38e579c)