kconfig: show an error message when defconfig is not found

When a non-existing defconfig is specified,
display an easy-to-understand message
(fake the error message on Linux Kernel):

  $ make foo_defconfig
  ***
  *** Can't find default configuration "confis/foo_defconfig"!
  ***

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Stephen Warren <swarren@nvidia.com>