kbuild: change the top Makefile to more Kbuild-ish structure

This commit changes the top Makefile to handle various targets
nicely.
Make targets are divided into four categories:

 - mixed-targets
     We can call a configuration target and build targets
     at one command line like follows:
     $ make <board_name>_config u-boot

     They are handled one by one.

 - config targets
     <board_name>_config

 - no-dot-config-targets
     Targets we can run without board configuration such as
       clean, mrproper, distclean, TAGS, %docs, etc.

 - build targets
     The other target which need board configuration.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>