Makefile: don't use parallel make when calling back into buildroot

The source-check / external-deps make targets ends up calling recursively
into buildroot's Makefile, causing make to display a warning:

make[2]: warning: -jN forced in submake: disabling jobserver mode.

We don't support toplevel parallel make, so get rid of the warning using
MAKE1 instead.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>