kbuild: Fix a false error of generic board support

Before this commit, make terminated with an error
where is shouldn't under some condition.

This bug happened when we built a board unsupporting
generic board right after building with generic board.

For example, the following sequence failed.
(harmony uses generic board but microblaze-generic does not
support it)

  $ make harmony_config
  Configuring for harmony board...
  $ make CROSS_COMPILE=arm-linux-gnueabi-
    [ Build succeed ]
  $ make microblaze-generic_config
  Configuring for microblaze-generic board...
  $ make CROSS_COMPILE=microblaze-linux-
  Makefile:488: *** Your architecture does not support generic board.
  Please undefined CONFIG_SYS_GENERIC_BOARD in your board config file.  Stop.

We had to do "make clean" before building the microblaze board.

This commit fixes this unconvenience.

Move generic board sanity check to "prepare1" target,
which is run after generation of include/autoconf.mk.

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