core/br2-external: properly report unexpected errors

core/br2-external: properly report unexpected errors

Unextected error in the br2-external script are properly caught, but
they are not reported properly, and we end up in either of two
situations:

  - the .br2-external.mk file is not generated, in which case make will
    try to find a rule to generate it (because the 'include' directive
    tries to generate missing files);

  - the .br-external.mk file is generated but does not contain the error
    variable, and thus the build might not get interrupted.

We fix that by using a trap on the pseudo ERR signal, to emit the error
variable on unexpected errors.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>