[coreboot] Flag Days reminder

[Stefan Reinauer]

Hi,

I just added two new items to our "upgrade guide for developers" called
"Flag Days":

http://www.coreboot.org/Flag_Days

However, it seems we neglected this practice for roughly 140 revisions
between r5911 and r6149. If you made any changes that will require
people update their uncommitted work, please go and add the items on
that page with a small description on what to do.

Thank you.

Stefan