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