kconfig: refactor .gitignore

Add '/' prefix to clarify that the generated files exist right under
scripts/kconfig/, but not in any sub-directory.

Replace '*conf-cfg' with '[gmnq]conf-cfg' to make it explicit, and
still short enough.

Use '[gmnq]conf' to combine gconf, mconf, nconf, and qconf.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>