build:arm allow to store build settings in a .settings file.

If present a file called .settings will be sourced when
calling the arm_sdimage.sh script. This allows to configure
the build setting for arm build without the need to specify
the flags each time arm_sdimage.sh is called.

For example to configure arm_sdimage.sh to target the
beaglebone put the following content in your .settings file.

BASE_URL=http://www.minix3.org/arm/beaglebone
FLAG=-DAM335X
CONSOLE=tty00

Change-Id: Ide0341a255bc7f41dc5c30d0a43e71b01a8ea55c
import netbsd ftpd

Change-Id: Id7a3dbd40a6f37c55bcbb0d1456301f60626298f