firmware/ipq-wifi: Extend for multi-chip boards

firmware/ipq-wifi: Extend for multi-chip boards

This package provides board-specific reference ("cal") data
on an interim basis until included in the upstream distros

While originally conceived for IPQ4019-based boards, similar needs
are appearing with three-radio devices. For some of these devices,
both a board-2.bin file needs to be supplied both for the IPQ4019
as well as for the other radio on the board.

This patch allows new or multiple overrides to be specified by:

  * Adding board name to ALLWIFIBOARDS
  * Placing file(s) in this directory named as
      board-<devicename>.<qca4019|qca9888|qca9984>
  * Adding
      $(eval $(call generate-ipq-wifi-package,<device>,<display name>))

(along with suitable package selection for the board)

At this time, QCA4019, QCA9888, and QCA9984 are supported.
Extension to other chips should be straightforward.

The existing files, board-*.bin, are "grandfathered" as QCA4019.

The package name has been retained for compatability reasons.
At this time it DEPENDS:=@TARGET_ipq40xx, limiting its visibility.

Build-tested-on: asus_map-ac2200, alfa-network_ap120c-ac,
    avm_fritzbox-7530, avm_fritzrepeater-3000, engenius_eap1300,
    engenius_ens620ext, linksys_ea6350v3, qxwlan-e2600ac-c1/-c2

Signed-off-by: Jeff Kletsky <git-commits@allycomm.com>