fastboot: Let "fastboot devices" work in all cases

Ignore ANDROID_SERIAL when requesting to list devices, or else it is
impossible to list devices without unsetting the env var. Even
  ANDROID_SERIAL=  fastboot devices
or
  fastboot -s "" devices
won't work.

Change-Id: Ie92f35bda852c6ecea4ebaefb75b8f18c10b6d0c