core: simplify removal of generated br2-external files

core: simplify removal of generated br2-external files

Now that all the br2-external generated files are named after the same
pattern, it gets easier to remove them all using a glob.

Furthermore, we're on the verge of introducing more such generated
files, so removing them at one fell swoop will be simpler too.

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>