Qt5: enable QML support

This patch enables experimental QML support in Qt5.

Only the QtQuick plugin is available yet.

Currently runs best on Linux and has some problems on NOVA and Fiasco.OC.

Fixes #979.
autopilot: improve detection of genode dir

If the script is executed with an obscure path (e.g.,
'../tool/autopilot'), just taking the argv0 string fails. Therefore, the
file path is now normalized prior to the directory detection.
autopilot: create only missing builddirs on --keep

This improvement prevents multiple execution of create_builddir on
existing directories.
base: spelling fix in signal/signal.h

ref #980
hw: provide Signal_context::submit

fix #980
Add exfat and fuse-ext2 includes to .gitignore
usb & panda: move EHCI-interrupt to board spec

ref #981
hw_panda: provide EHCI interrupt for USB driver

fix #981
TrustZone vmm: rename to tz_vmm due to ambiguity
hw_imx53: use same link address with or without TZ
hw_vea9x4: re-enable TrustZone support
tz_vmm: make scenario reproducible by everyone