kbuild: support simultaneous board configuration and "make all"

kbuild: support simultaneous board configuration and "make all"

This commit fixes two problems:

[1] We could not do board configuration and "make all"
    in one command line.

For example, the following did not work as we expect:
  $ make sandbox_config all
  Configuring for sandbox board...
  make: Nothing to be done for `all'.

[2] mixed-target build did not work with -j option

For example, the following did not work:
  $ make -j8 sandbox_config u-boot
  Makefile:481: *** "System not configured - see README".  Stop.
  make: *** [u-boot] Error 2
  make: *** Waiting for unfinished jobs....
  Configuring for sandbox board...

Going forward, we can do
  $ make -j8 sandbox_config all

This is the same as
  $ make sandbox_config
  $ make -j8

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