dropbear: add support for custom arguments through config file

dropbear: add support for custom arguments through config file

The default port 22 used by dropbear for its SSH connections is not always
desired. Dropbear accepts an option '-p' to set the port, but doing this was
not possible from the buildroot-provided init script.

One way to fix this is by adding a custom S50dropbear in a project-specific
rootfs overlay. However, this approach has the big disadvantage that bug
fixes or improvements in the default init script (i.e. in newer buildroot
releases) are not available (unless you manually port these changes each
time you upgrade buildroot).

Another solution is to modify the default init script from a
project-specific post-build script. However, this is fragile because you'd
have to sed some line but this line may change in later buildroot releases.

Yet another solution is to change the default port at build time, by
patching the options.h header file in the dropbear sources. This was
proposed with a patch [1] before, but not accepted.

This patch implements another solution, hinted from the discussion in [1]:
the default init script now sources a config file /etc/default/dropbear, in
which the user can set the variable DROPBEAR_ARGS. This is similar to the
S81named init script in the bind package. The config file would be added to
a project-specific rootfs overlay, a custom skeleton, or created from a
post-build script.
This approach has the advantage of being simple and non-intrusive, without
any code duplication or fragile script modifications.

[1] http://lists.busybox.net/pipermail/buildroot/2013-November/083165.html

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>