base-files: mtd_get_mac_ascii bugfix

The mtd_get_mac_ascii utility function was broken. This fixes it.

  - Remove the superfluous include of /lib/functions.sh. The
    function is already in that file so it is pointless,
  - only use a variable if the whole key word matches,
  - don't try to process the MAC address if it is empty,
  - use 'tr' to canonicalize the MAC address,

Signed-off-by: Daniel Gimpelevich <daniel@gimpelevich.san-francisco.ca.us>
[juhosg: add more fixes, update commit message]
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@38690 3c298f89-4303-0410-b956-a3cf2f4a3e73