core: rename generated .br-external.mk file

core: rename generated .br-external.mk file

Now that the two (all of them!) br2-external related files are generated
in the same location, it makes sense they are named after the same
pattern.

When initial support for (then single) br2-external trees was added back
in a4239f7fd1 (core: introduce the BR2_EXTERNAL variable), it was not
clear-cut why that file was not named with a br2 prefix.

So rename it now.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Vadim Kochan <vadim4j@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>