CMake: Add FindseL4_libs.cmake module

This adds support for importing this project via
`find_package(seL4_libs)` if the project exists in the
CMAKE_MODULE_PATH.

FindseL4_libs.cmake

diff --git a/FindseL4_libs.cmake b/FindseL4_libs.cmake
new file mode 100644
index 0000000..adf44d9
--- /dev/null
+++ b/FindseL4_libs.cmake
@@ -0,0 +1,21 @@
+#
+# Copyright 2019, Data61
+# Commonwealth Scientific and Industrial Research Organisation (CSIRO)
+# ABN 41 687 119 230.
+#
+# This software may be distributed and modified according to the terms of
+# the BSD 2-Clause license. Note that NO WARRANTY is provided.
+# See "LICENSE_BSD2.txt" for details.
+#
+# @TAG(DATA61_BSD)
+#
+
+set(SEL4_LIBS_DIR "${CMAKE_CURRENT_LIST_DIR}" CACHE STRING "")
+mark_as_advanced(SEL4_LIBS_DIR)
+
+macro(sel4_libs_import_libraries)
+    add_subdirectory(${SEL4_LIBS_DIR} seL4_libs)
+endmacro()
+
+include(FindPackageHandleStandardArgs)
+FIND_PACKAGE_HANDLE_STANDARD_ARGS(seL4_libs DEFAULT_MSG SEL4_LIBS_DIR)