Unify framebuffer driver binary names (fix #720)

Introduce 'framebuffer' SPEC variable to distinguish hardware framebuffer
drivers furthermore from the SDL framebuffer driver used by Genode/Linux.
Implement i.MX53 gpio driver
Unify GPIO driver binary names to 'gpio_drv'

Introduce 'gpio' SPEC variable to easily support a corresponding driver within
run scripts.
Implement i.MX53 framebuffer driver
Implement platform driver for i.MX53 platform

Move clock and reset controller functionality out of framebuffer driver
into platform driver.
Rework i.MX53 framebuffer driver

* Simplify IPU register definitions using templates
* Distinguish between i.MX53 QSB and SMD board in driver
* Support IPU specific overlay mechanism by framebuffer session extension
Double buffering for i.MX53 fb driver (fix #721)

Enable optinal support for double buffering in the i.MX53 framebuffer
driver. This prevents flickering in certain scenarios, where applications
directly render in the framebuffer dataspace given by the driver.
Remove superfluous pl11x target.mk file (fix #722)
base-linux: Fix use-after-free problem of ds fds

Fixes #717
base-linux: Fix condition in ~Reply_channel

Issue #717
Avoid leakage of signal-source reply capabilities

Issue #717
lx_hybrid: Safety check when freeing thread

Try to free the metadata of a thread only if created by Genode. The code
did not cause any actual problems. I just stumbled over the inconsistency
while reviewing the code.
Loader: Free entries of parent-service registry

The parent-service registry is populated on demand by the
'Loader::Child' whenever a prior unknown service is requested. Since the
number of parent services is limited, we expect the registry to settle
after a while. However, each loader session has a private instance of
a parent-service registry. So when creating and destroying loader
sessions, parent registries will be populated again and again. We
have to make sure to discard the entries along with the destruction
of a loader session to avoid the leakage of memory.

Issue #717
libc_lwip: accept 'AF_INET' sockets only

lwIP only supports the 'AF_INET' domain, but doesn't check the 'domain'
argument of the 'lwip_socket()' function.

This patch avoids an error message from lwIP when the Arora browser
tries to connect a socket of the 'AF_LOCAL' domain.

Fixes #732.
Create the context area RM session at startup

With this patch, the thread context area RM session gets created at
program startup to have the thread context area's virtual address range
reserved right from the beginning.

Fixes #734.

This patch implies that all Genode processes try to create an RM
session. So a route to the RM service must be present even for processes
that have only a single thread. Hence, the patch contains an update of
affected components.
hw & core: provide env_context_area_rm_session()

Ref #734
Add missing route to lx_uid.run
Add missing route to chroot_loader test

Additionally, we do the cleanup of the remains of a previously failed
run prior creating the run directory. Otherwise, the directly creation
may fail because of a pending bindmount.
Adapt chroot.run to changed timer