As said in a related github discussion thread [1], OpenWrt is so small a
system that

 - "airmon-ng check kill" does not have much to check
 - airmon-ng takes up too much space for not that much benefit (bringing
   up monitor interface with just iw should be enough for at least b43
   and ath9k drivers)

Airmon-ng depends on a lot of utility binaries to run and these
dependencies increase the installation size.  With this commit we can
provide a working airmon-ng and a smaller aircrack-ng package.

Dependency spec of airmon-ng is also reworked to

 - select usbutils only if USB feature is available
 - select pciutils only if PCI feature is available

While doing the split, this commit also contains a few other minor
changes

 - Use SPDX identifier for license name.
 - Add PKG_LICENSE_FILES definition.
 - Set PKG_MAINTAINER (MAINTAINER variable will take value from it).

 [1] https://github.com/openwrt/packages/pull/1693

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>