mt76: split kmod package

This patch splits `kmod-mt76` into three separate packages:
`kmod-mt76-core`, `kmod-mt76x2` and `kmod-mt7603`. By making
`kmod-mt76` a metapackage containing these new packages,
the previous behaviour of including all drivers and firmware
is left unchanged, unless explicitly unselected in
`DEVICE_PACKAGES`.

This splitting is especially beneficial for devices with
small flash chips, since the `kmod-mt76` package currently
requires ~160K on squashfs (after compression).

Signed-off-by: Joseph C. Lehner <joseph.c.lehner@gmail.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name> [cleanup]