ath79: use board name in LED migrations

ath79: use board name in LED migrations

Several devices added to LED migration script will just have their
(old) board name converted to tp-link.

By using a variable for this, the amount of code in the migration
script can be reduced and the chance for typos is reduced.

This patch also introduces the marker for beginning of a pattern
"^" to the regex, so the match is more specific.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
(cherry picked from commit 6b0eb84336edc995f4723f0a005507f158d3e895)