For the SPL configuration, "make <dir>/<target>" is used.
Here,
  <dir> is either "spl" or "tpl"
  <target> is one of "config", "menuconfig", "xconfig", etc.

This commit adds two checks:

[1] If <dir> is given an unsupported subimage, the configuration
    should error out like this:

  $ make qpl/menuconfig
  ***
  *** "make qpl/menuconfig" is not supported.
  ***

[2] Make sure that "CONFIG_SPL" is enabled in the ".config" before
    running "make spl/menuconfig.  Otherwise, the SPL image
    is not built at all.  Having "spl/.config" makes no sense.
    In such a case, the configuration should exit with a message:

  $ make spl/menuconfig
  ***
  *** Create ".config" with "CONFIG_SPL" enabled
  *** before "make spl/menuconfig".
  ***

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Suggested-by: Simon Glass <sjg@chromium.org>