kbuild: do not create built-in objects for external module builds

'make M=/path/to/your/external/module' creates a pointless built-in.a
in the top of the external module directory because KBUILD_BUILTIN is
set to 1.

Clear KBUILD_BUILTIN when we are building external modules so that
'make M=...' and 'make M=... modules' work equivalently.

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