Build: split COBJS value into multiple lines

This change is in preparation for condtitionial compile support in the
build system.  By spliting them all into seperate lines now, subsequent
patches that change 'COBJS-y += ' into 'COBJS-$(CONFIG_<blah>) += ' will
be less invasive and easier to review

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>