Makefile: clarify comment on .br-external handling

The comment language may lead the reader to think that .br-external is removed
whenever BR2_EXTERNAL is not set in the command line. Make it clear that
BR2_EXTERNAL must be explicitly set to an empty value for .br-external to get
removed.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>