tooling: Fix gen_version_h to query git support for the correct repository

The initial git availability check ran in the kernel directory instead
of the Jailhouse source tree. Fix it by moving the cd before it.

Reported-by: Henning Schild <henning.schild@siemens.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Documentation: Add Doxygen infrastructure

We diverge from Linux kernel style and use Doxygen as our code
documentation generator, see [1] for the reasoning. This adds the
required build infrastructure. Run "make docs" to trigger it (not
automatically done via other targets).

[1] http://thread.gmane.org/gmane.linux.jailhouse/1281

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
core: Prefix arch-specific PCI functions properly

Add the "arch_"-prefix to pci_suppress_msi, pci_update_msi and
pci_update_msix_vector. This clearly signals that those functions have
to be implemented by the architecture support.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
core: Completely document PCI subsystem

Convert existing kernel-doc comments to doxygen style and add missing
functions and structure descriptions.

No functional changes.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>