dropbear DEVICE variable is ignored when sourced from conf.d

[Andrei Pozolotin]

1) this is what "/usr/share/initramfs-tools/init" does:
```
. /conf/initramfs.conf
for conf in conf/conf.d/*; do
 [ -f ${conf} ] && . ${conf}
done
. /scripts/functions
```

2) this is what "/usr/share/initramfs-tools/scripts/init-premount/dropbear" does:
```
. /scripts/functions
. /conf/initramfs.conf
configure_networking &
```

3) bug result: ```configure_networking ``` will not find a $DEVICE variable
if the $DEVICE variable is overriden from some ```/etc/initramfs-tools/conf.d/user-file```

4) I would hope this be somehow enforced for all scripts?

###

lsb_release -rd
Description: Ubuntu 14.04.2 LTS
Release: 14.04

###

apt-cache policy initramfs-tools
initramfs-tools:
  Installed: 0.103ubuntu4.2
  Candidate: 0.103ubuntu4.2
  Version table:
 *** 0.103ubuntu4.2 0
        500 http://us.archive.ubuntu.com/ubuntu/ trusty-updates/main amd64 Packages
        100 /var/lib/dpkg/status
     0.103ubuntu4 0
        500 http://us.archive.ubuntu.com/ubuntu/ trusty/main amd64 Packages