Makefile: simplify handling of common board configurations

Many boards don't need any special handling in the Makefile. Try and
provide as generic make rules for these as possible.  There are still
many areas where this does not work out really well, but the changes
show the direction we should take, and point out which boards or
architectures need further cleanup.

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