fix doxy target in root Makefile

the moving of 'documentation' to 'Documentation/' means that the doxygen
target in the main Makefile was broken as it couldn't find the directory.

Change-Id: If6c6d34110e683f38959571a03767fb472675f40
Signed-off-by: Nicky Sielicki <nlsielicki@wisc.edu>
Reviewed-on: http://review.coreboot.org/10445
Tested-by: build bot (Jenkins)
Reviewed-by: Idwer Vollering <vidwer@gmail.com>
1 file changed