ramips: purge default packages on MT7621

In order to have a smaller initramfs image remove all packages not
needed on all devices and add them explicitely for those actually
needing them. Also remove wpad-mini from ramips default package set
and add it to all sub-targets except for MT7621.
While at it reorder packages alphabetically and replace kmod-mt76 with
kmod-mt7603 and/or kmod-mt76x2 depending on the chip actually used on
a specific board.

Hopefully fixes FS#758

Signed-off-by: Daniel Golle <daniel@makrotopia.org>