kconfig: refactor conf_write_dep()

The if ... else inside the for-loop is unneeded because one empty
line is placed after printing the last element of deps_config.

Currently, all errors in conf_write_dep() are ignored. Add proper
error checks.

Rename it to conf_write_autoconf_cmd(), which is more intuitive.

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