xpp: fix PANIC for old dahdi_registration

* dahdi_registration writes multiple times to:
     /sys/bus/astribanks/devices/*/*/span
* In some race cases this resulted in corruption and eventual kernel
  panic.
* Until migration to "assigned-spans" is complete:
  - Accept and ignore multiple "dahdi registrations" from user-space.

Signed-off-by: Tzafrir Cohen <tzafrir.cohen@xorcom.com>
firmware: Honor DESTDIR when installing firmware.

This fixes a regression introduced first in release 2.9.1 with commit
(7ce849846502ff9d "firmware: Refactor by using build_tools/install_firmware.")
which prevents from installing the firmware in a location other than the system
root.

Bug: https://issues.asterisk.org/jira/browse/DAHLIN-337
Reported-by: Anthony Messina <amessina@messinet.com>
Signed-off-by: Tzafrir Cohen <tzafrir.cohen@xorcom.com>
[edited the commit message]
Signed-off-by: Shaun Ruffell <sruffell@digium.com>