config.mk cleanup: drop "-I$(TOPDIR)/board" entries

After the recent cleanups, a number of config.mk files consist only of
a "PLATFORM_CPPFLAGS += -I$(TOPDIR)/board" entry whih is not needed.
Remove such entries.  In most cases, that means that the whole
config.mk file can be removed.

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