Update links to flash-debian script now that Debian 7.2 is out

Debian 7.2 adds support for the TS-12x, TS-22x and TS-42x devices,
so there's no need to link to a local copy anymore.
Use HTTP rather than FTP links

Use HTTP rather than FTP links since HTTP is more common these days.
Remove note about QNAP flash issue that's been fixed in 7.2