ramips: allow custom trx magic for Arcadyan

ramips: allow custom trx magic for Arcadyan

This commit:
1. Renames beeline-trx recipe in mt7621.mk to arcadyan-trx. The recipe
   is necessary for:
   - MTS WG430223 (Arcadyan WG430223)
   - Beeline Smartbox Flash (Arcadyan WG443223)
2. Allows specify custom trx magic which is different for the routers
   mentined above.

Signed-off-by: Mikhail Zhilkin <csharper2005@gmail.com>
(cherry picked from commit 109c503bee9aed34ffb485a29af1e2ec6f3bb6b1)
[fix merging conflict in mt7621.mk]
Signed-off-by: Mikhail Zhilkin <csharper2005@gmail.com>