ramips: ex2700: actually remove kmod-mt76*

ramips: ex2700: actually remove kmod-mt76*

When building using the multiple devices option with per-device root
filesystem, only the meta package mt76 is omitted but not the
dependencies selected by the package.

Explicitly exclude all 3 mt76 packages, plus the metapackage.
Otherwise, these modules will be included in the build, wasting
a few hundred kilobytes.

Signed-off-by: Joseph C. Lehner <joseph.c.lehner@gmail.com>
[mention the root cause of the issue in the commit message]
Signed-off-by: Mathias Kresin <dev@kresin.me>