When build using libjaylink as git submodule, doxygen includes the
libjaylink files and complains for multiple 'mainpage' comment
block, one in OpenOCD and the other in libjaylink:
found more than one \mainpage comment block! (first
occurrence: doc/manual/main.txt, line 1), Skipping current
Exclude libjaylink submodule from doxygen.
Signed-off-by: Antonio Borneo <email@example.com>