Makefile: compile and link each module just once

Several source files need to be compiled and linked when one or more
config options are selected. To allow for easy selection in the
Makefiles yet to avoild multiple compilation (which costs build time)
and especially multiple linking (which causes errors), we use
"COBJS = $(sort COBJS-y)" which eliminates duplicates.

By courtesy of Detlev Zundel who suggested this approach.

Signed-off-by: Wolfgang Denk <wd@denx.de>