Add unified build directories to create_builddir

This patch makes the benefit of the recently introduced unified Genode
ABI available to developers by enabling the use of multiple kernels from
within a single build directory. The create_builddir tool has gained a
new set of kernel-agnostic platform arguments such as x86_32, or panda.
Most build targets within directories are in principle compatible with
all kernels that support the selected hardware platform. To execute a
scenario via the run tool, one has to select the kernel to use by
setting the 'KERNEL' argument in the build configuration
(etc/build.conf). Alternatively, the 'KERNEL' can be specified as
command-line argument of the Genode build system, e.g.:

  make run/log KERNEL=nova

This allows us to easily switch from one kernel to another without
rebuilding any Genode component except for the very few kernel-specific
ones.

The new version of the 'create_builddir' tool is still compatible with
the old version. The old kernel-specific build directories can still be
created. However, those variants will eventually be removed.

Note that the commit removes the 'ports-foc' repository from the
generated 'build.conf' files. As this is only meaningful for 'foc',
I did not want to include it in the list of regular repositories (as
visible in a 'x86_32' build directory). Hence, the repository must
now be manually added in order to use L4Linux.

Issue #2190

repos/base-linux/etc/specs.conf

@@ -1,23 +1 @@
-
-SPECS += genode linux sdl
-
-#
-# Detect host CPU architecture
-#
-UNAME_MACHINE := $(shell uname -m)
-
-ifeq ($(UNAME_MACHINE),i686)
-SPECS += x86_32
-endif
-
-ifeq ($(UNAME_MACHINE),x86_64)
-SPECS += x86_64
-endif
-
-ifeq ($(UNAME_MACHINE),armv6l)
-SPECS += arm_v6
-endif
-
-ifeq ($(UNAME_MACHINE),armv7l)
-SPECS += arm_v7a
-endif
+SPECS += linux sdl