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:
	src/jtag/drivers/libjaylink/libjaylink/core.c:37: warning:
	found more than one \mainpage comment block! (first
	occurrence: doc/manual/main.txt, line 1), Skipping current
	block!

Exclude libjaylink submodule from doxygen.

Change-Id: I5e856817344c9f21f8c26f077a23c00b83cfbcb5
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/6000
Tested-by: jenkins