kbuild: refactor the check for missing config files

This commit refactors the check for missing configuration files, making
it easier to add more files to the list.

The format of the error message has been slightly changed, as follows:

[Before]

    ERROR: Kernel configuration is invalid.
           include/generated/autoconf.h or include/config/auto.conf are missing.
           Run 'make oldconfig && make prepare' on kernel src to fix it.

[After]

  ***
  ***  ERROR: Kernel configuration is invalid. The following files are missing:
  ***    - include/generated/autoconf.h
  ***    - include/config/auto.conf
  ***  Run "make oldconfig && make prepare" on kernel source to fix it.
  ***

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <n.schier@avm.de>
Tested-by: Miguel Ojeda <ojeda@kernel.org>