mwlwifi: add and use individual firmware packages

mwlwifi: add and use individual firmware packages

As each mvebu device only uses one of the firmwares provided by mwlwifi
package, it makes sense to put them in separate packages and only install
the one that is needed.

Current mwlwifi version's firmware sizes and usages by devices:
88W8864.bin  118776  caiman, mamba, cobra, shelby
88W8897.bin  489932  (none)
88W8964.bin  449420  rango

Changes by this commit:
 * indicate in title that mwlwifi also is driver for 88W8897 and 88W8964
 * remove mwlwifi package's firmware installation rules
 * add 3 new individual firmware packages (all depends on kmod-mwlwifi):
    - mwlwifi-firmware-88w8864
    - mwlwifi-firmware-88w8897
    - mwlwifi-firmware-88w8964
 * add firmware package to mvebu devices' DEVICE_PACKAGES accordingly

Signed-off-by: Johnny S. Lee <_@jsl.io>
[Add the used FW files to the PACKAGES of default image]
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>