kbuild: add outputmakefile to no-dot-config-targets

The target outputmakefile is used to generate a Makefile
for out-of-tree builds and does not depend on the kernel
configuration.

Signed-off-by: David Engraf <david.engraf@sysgo.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>