Add sdbfs library as submodule.

Provided by the fpga-config-space repository on OHWR. We are using the
'import' branch of the repository, which adds changes from many versions
of sdbfs spread across different projects.

.gitmodules

@@ -1,3 +1,6 @@
 [submodule "docopt.cpp"]
 	path = third_party/docopt.cpp
 	url = https://github.com/docopt/docopt.cpp.git
+[submodule "third_party/fpga-config-space"]
+	path = third_party/fpga-config-space
+	url = https://ohwr.org/project/fpga-config-space.git

third_party/meson.build

@@ -1,3 +1,11 @@
 docopt_lib = static_library('docopt', 'docopt.cpp/docopt.cpp')
 docopt_inc = include_directories('docopt.cpp')
 docopt = declare_dependency(link_with: docopt_lib, include_directories: docopt_inc)
+
+sdbfs_inc = include_directories(['sdbfs/lib', 'sdbfs/include/linux'])
+sdbfs_lib = static_library(
+  'sdbfs',
+  ['sdbfs/lib/access.c', 'sdbfs/lib/glue.c'],
+  include_directories: sdbfs_inc
+)
+sdbfs = declare_dependency(link_with: sdbfs_lib, include_directories: sdbfs_inc)

third_party/sdbfs

@@ -0,0 +1 @@
+fpga-config-space/sdbfs