ar71xx: base-files: shorten case statement patterns in lib/ar71xx.sh

ar71xx: base-files: shorten case statement patterns in lib/ar71xx.sh

At now there is no general rule about what part of (or whole) machine
name string should be used for board name assignment/detection and every
target handles this in a different way.

For most of the boards in ar71xx we already use only part of the string,
generally without the vendor name.

This shortens wildcards patterns in case statement for board name
assignment, wherever possible (e.g. where it won't be misleading).

Signed-off-by: Piotr Dymacz <pepe2k@gmail.com>