Add new 'fs_rom' service

The 'fs_rom' service provides files stored on a file system as ROM
modules via the ROM-session interface.

Fixes #606
New Input::Event::FOCUS, rename keycode to code

This patch introduces keyboard-focus events to the 'Input::Event' class
and changes the name 'Input::Event::keycode' to 'code'. The 'code'
represents the key code for PRESS/RELEASE events, and the focus state
for FOCUS events (0 - unfocused, 1 - focused).

Furthermore, nitpicker has been adapted to deliver FOCUS events to its
clients.

Fixes #609
Add 'Service_registry::remove_all' function

This function is needed when reconfiguring init.
Make 'config()' convenience utility more robust

This patch improves the config handling by falling back to a static
string (empty "<config />") if no valid config ROM module could be
found. This can happen initially, but also at runtime when the ROM
module dissapears, e.g., a ROM module accessed via fs_rom where the
corresponding file gets unlinked.
Init: handle reconfiguration

With this change, init becomes able to respond to config changes by
restarting the scenario with the new config. To make this feature useful
in practice, init must not fail under any circumstances. Even on
conditions that were considered as fatal previously and led to the abort
of init (such as ambiguous names of the children or misconfiguration in
general), init must stay alive and responsive to config changes.