Don't block the upstart daemon if it doesn't exist

Check if Upstart exists before blocking the daemon.  Not all dpkg based
OS's have Upstart installed so this should help increase compatibility.

Closes-Bug: #1251949
Change-Id: I2dcb1ff3641778b5653ca5762a728398adb57da1
Make pxe_mac accurate in two common cases.

If we are booting pxe booting using syslinux, and it has IPAPPEND 2 in
the boot stanza, then it will append the mac address of the device we
are booting from to the kernel parameters where we can get at it
pretty easily.

If we are booting physical hardware via UEFI over the network, we can
rely on the BootCurrent EFI variable to point at the boot entry for
the NIC we booted from, which will include the MAC address of that
nic.

If neither of those cases are in play, we can just fall back to the
all-physical-devices-with-links code.

This currently uses the Bash 4 support for associative arrays to handle
the netboot-in-UEFI case, if needed I can rewrite it to be Bash 3 compatible.

Change-Id: I5e50e30c60d6d732a09ab61251cbb9be08bb6113
Merge "Don't block the upstart daemon if it doesn't exist"